There are a number of mods that claim to improve FPS in Minecraft. My experience is limited to OptiFine (an updated version of Optimine from around version 1.2) on 1.8.1, but I do see a very significant improvement in my FPS. Although it's a bit non-specific, I have seen my FPS on a Mac go from completely unplayable to very smooth.
OptiFine (Minecraft Forum Link) reworks a number of rendering details to improve Minecraft performance (+20fps). Many people go crazy about it on the forum, others say that it just doesn't work so YMMV.
Last I heard, Notch was actually including some of these optimizations into Minecraft proper.
Keep in mind that any Minecraft mod that's not a skin requires you to patch the Minecraft code itself and thusly may not be compatible with other mods or your Minecraft version.
Update: According to Notch's blog post about changes coming in 1.6, the torches-on-fast-leaves bug is getting fixed:
- Fixed buttons and levers being placeable on leaves in “fast graphics” mode
This means that monsters can no longer spawn on leaves, either, as they are always considered transparent, even when using Fast graphics.
Monsters can spawn on any solid, non-transparent block which is at light level 7 or lower.
Signs, torches, rails, and the like are ignored when spawning monsters; they'll spawn on the block underneath just fine. This is why one-block-wide railways need to be lit.
Transparency means whether or not light passes through a block rather than whether you can see through it. Half-blocks, doors, stairs, and glass all count as transparent and prevent monster spawning. Leaves, oddly, count as transparent only when Fancy graphics are enabled. An easy way to test a block for transparency is to try placing a torch or sign on it — neither can be placed on transparent blocks. (Meaning that torches can be placed on leaves with Fast graphics, but not with Fancy.)
Torches put out light level 14, so monsters can't spawn within 5 blocks of a torch orthogonally or 3 blocks diagonally. If you put your torches in a diamond pattern, they can be up to 9 blocks apart, but it's best to move them closer than that on uneven terrain. (I use 5 myself.)
If you like, you can actually torch your treetops by changing your graphics to Fast, placing the torches, and switching back. Your torches will stay rooted, even though you wouldn't normally be able to place them that way. Personally, I think it makes them look rather cheerful. :-)
Best Answer
Creepers can certainly get close enough to arm through cobblestone wall, but that's not the problem; they can even get close enough through a two-layer thick wall of obsidian; their arming range is a full three blocks. And yet, even a door is usually thick enough to stop them.
Creeper arming is not just based on distance, but also visibility. Cobblestone Walls are a block and a half tall for collision purposes, but not for Creeper line-of-sight. If the walls are only one layer tall, creepers have a clear line of sight over it and can still arm and fire.
Note that even transparent blocks like glass, glass panes, etc. block creepers' arming. And they can't see through the holes that form in Cobblestone walls more than one layer high; the blocks all still count as completely solid. However, the blocks aren't completely solid when they run up to most other blocks. So, while a glass wall,
a cobblestone wall,
and doors
are all fully able to stop them from arming, placing either glass or a door next to the walls gives them a gap to see you through:
Additionally, you can stop them from arming by getting much higher ground on them. If the top of their head is at least a full block below your feet, they won't arm.