well if it's american.. you can go 6600gt.. though thats starting to outdate.. if you wanna play bad games like half life 2.. LOL bad game.. naw it's not but anyway, low grade games like such.. then you'll be fine. upcoming stuff like fear, bf2 (out) so on, wont work on ur computer though
you can go for an amd 3000+ with 1 gig of ram and a decent pci e board with 6600gt for that money 600.. IF it's american.. and IF ur good with barganing..
happy gaming champ