What does upgrading Pyromancy Flame do

You are correct in saying that stat scaling doesn't really improve at all, beyond a minor improvement from ascensions (a sword might go from E --> D for example). In general, the stat classes that each weapon has in the game won't change much, even after being ascended fully.

Compared to Demon's Souls, upgrading a weapon to its max in Dark Souls provides less benefit because of this.

The bonus damage from stat scaling does increase for +n weapons, albeit it's a minor, linear increase (for example, a Battle Axe from +0 to +2 made its bonus damage from stats go +20->+22->+24). Thanks Tacroy

If you need to know when to replace your Drake's Sword, I would say get rid of it when you have high enough stats in one of the "damage" stats (Strength/Dexterity/Faith) and you have an adequate amount of resources to reinforce a scaling weapon to its max.

Stats

What they do

• Vitality determines your health.
• Attunement determines how many spell slots you have.
• Endurance determines your stamina and equip load.
• Strength/Dexterity determines your attack power with weapons that scale with strength/dexterity, and is also required for wielding certain equipment.
• Resistance increases your defense.
• Intelligence increases the effectiveness of sorceries, and increases the damage of weapons that scale with intelligence. Certain spells and equipment have an intelligence requirement.
• Faith determines the effectiveness of miracles, and increases the damage of weapons that scale with faith. Certain miracles and equipment have a faith requirement.

Stats important for a Tank build

• Vitality: Vitality increases your health and as such is, obviously, very important to any Tank build. However you shouldn't increase this too far beyond 40-points since the amount of health gained per level decreases significantly after it reaches 40 points.

• Endurance: Endurance increases your stamina and equip load. For a tank character, this will have a large impact on how often you can attack and block. It also determines your maximum equip load, which determines how heavy the equipment you wield can be (armour and all).

  As a tank you'll want this to be high enough so you can use the armour and weapons for your build. Similar to vitality, endurance becomes less useful after 40 points since stamina will stop increasing, and only your equip load will increase. I would suggest investing no more than 40 points into it because of this, but you will probably be able to get by only investing around 30 points.

• Strength: This will be your offensive stat. No matter how good your defense is, it won't mean anything if you can't deal any damage to your enemies. Furthermore, a high strength is required to wield great shields, which allow you to block almost any attack, whereas normal or small shields can only block some attacks. Therefore great shields are vital to any tank build.

  Strength also will allow you to wield the Drake Sword. Although not that useful late game since it doesn't scale, it is very useful early game. Using the Drake Sword will allow you to invest more points into vitality and endurance before you need to invest points into strength to keep your damage up.

  How many points you invest here will depend on what weapon you decide to use, which I'll touch on later, but I wouldn't invest anymore than 30 points in here until you boost your vitality and endurance up to around 30 - 40 points.

A stat I didn't list which you may find unusual is resistance, which increases your defense. The reason for this is that the further into the game you go the increased defense offered by resistance becomes negligible, making any points investing here wasted.

Finally, how many points you invest into dexterity will depend on which weapon your wind up using. You'll want to invest just enough points into dexterity to use your desired weapons. I'll be discussing weapons later.

(reference: http://darksoulswiki.wikispaces.com/Stats)

Equipment

Rings:

• Havel's Ring: Havel's Ring increases your maximum equip load by 50%. This is extremely useful for a tank character. Not only will it allow you to wield heavier armour, but will also allow you to get away with investing less points into endurance, thus allowing more points to be investing into Vitality and Strength.

• Ring of Favor and Protection: Another vital ring for any tank build, and is an excellent ring for any build. The ring increases your maximum health, stamina, and equip load by 20%. As you can imagine, this is incredibly useful for a Tank. Not only because it lets you take more damage, but, like Havel's Ring, can allow you to get away with not investing as many points into Vitality and Endurance so you can put more points into strength.

  The major downside to this ring is that if you ever remove it, it's destroyed. So never take it off. Due to this you will need to make sure your equip load is high enough to wield your equipment without Havel's Ring, since there are a few points in the game where you are forced to wear a certain ring in order to proceed.

Weapons and Shields:

• Eagle Shield: The Eagle Shield has the lowest strength requirement of all the great shields, and also the lowest weight. It's stats are comparable to other great shields, so I would suggest just sticking with this great shield for most of the game.

• Zweihander: Found fairly early on in the game, the Zwiehander has a fairly high damage output. Although other weapons may surpass it late game, the benefits of the Zwiehander is that it can deal high damage without investing a lot of points into strength. Compared to other ultra greatswords it has fairly low weight and strength requirement, which also makes it fairly attractive for a tank.

• Claymore: Although the damage output of the Claymore is considerable lower than the Zweihander, so are it's strength requirement and weight. It also uses up less stamina per swing than the Zweihander. These things will probably make it more desirable early game, when your strength and equip load aren't as high.

Armour

Before I discuss armour, I should give you some details on what Poise is. In a nutshell, poise determines how many hits you can take before staggering. A higher poise results in you taking more hits before staggering. Due to this having an armour with high poise is very important for a tank build, and this is reflected in my armour suggestions below.

Although all the details of poise are still not known, a user from gamefaqs was nice enough to do some extensive testing on poise and made a thread about it. Although I would suggest reading the thread for more detail, the key things to take away is that poise behaves like a meter, similiar to stamina. Every time an enemy hits you your poise meter decreases, and certain hits will consume more poise than others. Once this meter hits zero, your character will stagger. This meter does recharge over time, but the exact value is unknown.

• Giant Armor Set: When fully upgraded, The Giant Armor Set has the highest physical defense in the game, and it has fairly high poise as well. However it requires Twinkling Titante to upgrade, which makes upgrading this armour expensive. You also need to buy this armour from a merchant, making it even more expensive.

• Black Iron Set: The defense and poise are both fairly high, which makes it excellent armour for a tank to use. Although it has less defense and poise than the Giant Armor Set, it's resistances are all higher, making it the preferred choice against enemies using a lot of elemental damage. The Black Iron Set also requires Twinkling Titante to upgrade, making upgrading it expensive. However, unlike the Giants Set you don't need to buy it.

• Havel's Set: Although it's defense may be lower than a fully upgraded Black Iron Set, it can't be upgraded, and therefore it will surpass the Black Iron Set in defense until you can afford to fully upgrade the Black Iron Set. The poise of Havel's Set is also considerable higher than the Black Iron Set, so you'll be able to take more hits before becoming staggered. It does weigh more than the Black Iron Set, but assuming you're wearing both the rings I suggested this shouldn't be a problem.

Starting Class

• Knight: The Knight starts with the highest vitality of all the classes, and it's armour also has the best defense. The Knight's starting shield also has 100% physical damage reduction, which makes it very useful until you find a great shield later in the game. It's strength and dexterity are both good as well, coming in at 11 points each. It's endurance is 10 points, which isn't bad, but it isn't great either.

• Warrior: Although not as suited to be a tank as the Knight, it does have higher strength, dexterity, and endurance than the knight. It's armour also has less defense than the knight, but it is also lighter, which will allow you to wield heavier equipment until you get your equip load up higher.

• Bandit: Another excellent choice would the Bandit. It has the highest strength and endurance of all the classes, and it's vitality is fairly high as well. However, it's starting armour is inferior to that of both the Knight and the Warrior, and it's dexterity is below average. The low dexterity will limit the weapons you are able to use early game, since you'll be forced to invest some points into it.

You can't go wrong with any of the above classes, although my personal recommendation would go to the Warrior. The Knight may surpass the warrior in early tanking ability, and the Bandit will surpass the warrior in pure damage, but the warrior has the most well rounded stats of the three overall.