If you want to play the majority out of games out there, buy the Athlon 3200+ (note, it's not 3.2 GHz, rather 2.something)
The primary purpose of Mac's aren't gaming, anyhow. You will also find that games depend a lot on the graphics card, especially nowadays with fragment programs (pixel/vertex shaders), where many graphics processing functions are being done by ALU's on the graphics card itself instead on the processor.