I'd love to have a new CPU... Really, my current one is the bottleneck of my PC in several games now, GTA IV amongst them. I'm almost certain that it might improve performance in other places as well. (Good god, i know that this can be misread as... You know, but that is not what I'm talking about. >.>)
A new GFX card wouldn't be bad either, though it isn't on the list of top priorities. My GeForce 8800 has served me quite well so far... And if my older brother gets a new one, i can run two 8800's in SLI. And that would, as far as i know, be some pretty darn serious graphical processing power.
It's mostly the CPU that's pressing though, but that will also require a new motherboard, unfortunately. I suppose that's on my list as well then.