Doesn't make a difference. I have yet to come across a game that is a 64bit program, thus they can't use anymore than about 3.5GB of RAM.
I don't game on my Mac much anyway. I've got a PC rig for that. Apple's inability to keep up with the latest OpenGL drivers hinders the performance of games on OS X (not to mention they are damned expensive unless you get Steam copies).
It's really a shame, as Mac gaming is finally starting to gain traction with Steam and Apple putting powerful GPUs in their high end lines.
Look at the iMac compared to PC AIOs. Find me a PC AIO with atleast a GTX 660M.
However, Bootcamp can be partly to blame. Since it's so easy to just load up windows and re-boot, there probably isn't a rush at apple to improve gaming performance in OSX. Civ 5 runs like sh-t on mac?(Which it does BTW) Just re-boot into windows and poof 60 seconds later I'm playing Civ 5 natively on windows.
My 'gaming rig' is literally the windows side of my MBP.(The GT 650M coupled with the i7 quad is really a monster, Everything I throw at runs on ATLEAST high settings smoother than silk, if not max.)