With a PCI 5200 you will likely be limited by your video card. CPU won't matter much.
As a gamer myself, I notice no difference between my A64 3000+ at 1.8GHz or overclocked to 2.5GHz in the games I play (WoW, Rome:Total War, and Battlefield2), even with a 7800GT. Most games are not terribly CPU dependent. UT2004 and HL2/CS:S are about the only games that require significant CPU power, even those are quite playable on lower end CPUs.
If you want a CPU to fit in your particular motherboard then you have to tell us what kind of socket/motherboard you have. Most people around here though favor the low end A64 S939 chips for gaming (3000+ / 3200+) because they are adequate stock, cheap, and overclock pretty well too.