The Cat 4.12 offers a slight improvement overall compared to the previous sets, and it may well show more than a handful fps improvement in some games (e.g., KOTOR and other OpenGL games, Doom 3, Half-Life 2, maybe other newer games). I haven't heard of any big issues with the 4.12, whereas some of the previous 4.x series had various crashing issues with KOTOR and Wolf:ET for sure. Anyway, I'm happy with the 4.12 in my very short time with it.
Normally I'd say leave well enough alone, but driver updates are fairly simple to do and undo. I'd give 'em a try if you feel performance is a little low in some titles. But I'd also download a 3.6/3.8 package in case the the 4.12 gives you trouble.