I've referred to this for my source on necromancer skills: http://classic.battle.net/diablo2exp/skills/necromancer-curses.shtml
Lower Resist decreases an enemies elemental resistances and Decrepify decreases their physical resistance. Unfortunately, bone spells do magic damage, so I don't think either curse improves your bone spell damage (as Grace Note pointed out). They are only useful for your minions; Lower Resist helps your skeletal mages and Decrepify helps your skeletons. Also, if you are using Revive then those minions will do more damage when you use those curses (most monsters don't do magic damage).
Unless you totally focus on them and skills that synergize (see my sidenote), Bone Spear and Bone Spirit are ineffective because they do not scale up in Hell difficulty. However, Bone Prison and Bone Wall do. Also curses and revives scale up because they are based on the strength of monsters in Hell.
For a summoner, playing heavily to revive is a good plan. Look at some revive necromancer guides, this being a short example: http://diablo.incgamers.com/forums/showthread.php?t=707530. I recommend maxing Revive and Iron Golem, so that you have plenty of summons to work with and a golem for killing monsters to get that first revive. I also think you're better off replacing Bone Spear with Bone Prison and Iron Maiden. Then you can lock monsters in with your minions and have them damaged by both their own attacks and your minions, while boosting with other curses you have. Iron Maiden has the nice property that an increase in skill level increases the damage taken, not the spell radius like many other curses. Bone Spirit is appropriate to some extent for helping your Iron Golem take down monsters, but certainly not worth focus.
Sidenote:
To get an idea of how much damage a pure bone necromancer does, I'm in Hell difficulty with my necromancer right now using an all bone build, around level 76, roughly these skills (I can update values tonight when I check):
- Teeth: 10 (about +6 with items)
- Bone Spear: 20
- Bone Spirit: 20 (about +4 with items)
- Bone Wall: 10
- Bone Prison: 20
- Some other misc. skills
And I only do over 3000 damage per spirit. That's very little in Hell difficulty.
The dolls/bone fetishes do an explosion, similar to the necromancers "corpse explosion" skill. It deals a huge amount of physical damage.
If you're a ranged player it should be easy to snipe them from the distance. If you're a melee character, go for -% physical damage gear, like Stormshield, Rockstopper, Vampire Gaze or other to reduce the damage. Also try to aim for high health (obviously).
AFAIK assassins can also block the explosion when they have a high "Weapon Block" skill. Still not something I would try though (at least not in hardcore mode :))
Best Answer
As soon as you're in Hell difficulty, you'll only see Games that are on that difficulty. You can create a Nightmare game by yourself any time however... or ask a friend who is in Nightmare difficulty if he could tell you the name of an open NM game.