I agree with the poster (skinnylegs, I think) who mentioned a different "feel" when gaming on a PC versus a Mac. While I'm not sure I can clearly state what the difference in "feel" might be, I can say that my PCs tend to be far more responsive while gaming than my Macs. Granted, neither platform is state-of-the-art in my house due to financial restrictions, but it seems like the PCs are quicker at 3D clock-for-clock. Maybe it's OSX's multitasking nature getting in the way, maybe it's OpenGL vs. DirectX, or maybe it's some low-level architectural quirk. Regardless, my PCs are the gaming systems and the Macs are productivity systems. Both run great for what I need them to do.
the PC: 800MHz AMD Duron, GeForce2GT, 512MB RAM, XP
the Mac: 733MHz G4 PowerMac, Geforce2GT, 756MB RAM, OS 10.3
PC was more responsive on Halo, WOW, AoE2, & SW Battlegrounds, which are all of the Mac games I tried before diverting my gaming budget to the PC side. (I've since built faster game systems)
Aside: has anyone tried running games natively in Linux? (not under Wine or other emulation). I've heard a couple games (EVE & ?) can be built and run on Linux. Reason being, I'd love to continue building speedy gaming PCs, but find some way to dump Windows for obvious reasons. (might take us back to OGL vs D3D again. ha)