games will be little to no different on most cpu's tbh, for gaming the best upgrade will always be a gfx card so spend as much as you can possibly afford there but don't go overboard. for instance the difference between a x1900xt and a xtx are minimal and not worth the cost, so i'd say get a x1900xt/7900gt/7900gtx(memory being 512 instead of 256mb on the gt model) or if you can afford it the 7950gx2 is a freaking beast of a card. a big question is what monitor do you have and what resolution. crt/tft that can do 1920x1200 with ease, the 7950gx2, maxed at 1600x1200 then any of the others will do, maxed at 1280x1024 and you won't really push any of those cards so either upgrade the monitor and get one of those cards, or just get something along the lines of a x1800xt or so and save the money.
conroe does pretty much spank the ath 64 upside down but 99% of all gaming you will do will be very gpu limited and barely within the limits of most cpu's let alone a really good cpu.
EDIT:-,. also most ath 64's will at least overclock a bit, if you have a decent cooler, you can probably get 2.5Ghz at least out of it i woulda though, never had an issue getting 2.5Ghz on any ath 64 across a large number of them now, thats more than enough cpu juice for any gaming right now. get the card, definately most important thing.
