As LordSplodge said, it depends what model you get.
I'm using a MacBook with 2GHz Intel C2D, 4GB DDR3, NVIDIA 9400M and a bootcamped copy of XP 32bit. It's plenty for me. I'm not sure about frame rates but it'll run Battlefield 2 on high settings with no problems, L4D 1&2 on medium settings and Fallout 3 (and to a lesser extent, Oblivion) on low settings.
Not the highest of fps but they're all perfectly playable and they don't look terrible either. To be honest, this machine is capable of more than I imagined it would be.
That should give you some idea of what to expect from even an entry level Mac.