well you got 3 different answers because you asked an ambiguous question. telling us that its a gaming rig makes it clear.
magouster is on the right path.. you'll need at least another 512mb of ram, and then a better gfx card. (a better gfx card will give you a bigger boost than a better cpu)
if you're dandy with mail in rebates, here's one route you can go:
btw.. i'm assumign you have pci-e slot... since you said you're considering a 7600.. i believe its made exclusively for pci-e (not sure about that..)
anyways.. this is what i would get
$116 after mail in rebate.
$42 after mail in rebate
total after mail in rebate: $158
then add shipping and possibly tax..