If you've been searching for answers to questions like these, I think Rob Griffiths over at Macworld might have cut your googling short with MacBook gaming: A graphics concern? Rob investigated this whole 'integrated graphics card' issue and found that the MacBook can perform surprisingly well, as long as you max it out with as much RAM as you can afford. Testing an unofficial Universal version of Quake 3 (while old, it is fairly 3D-intensive), the MacBook cranked out 52 fps with 512 MB of RAM (which is already a great stat), but once he maxed the machine to 2 GB, Quake 3 offered up 98 fps.
Rob explores gaming performance on the MacBook with a wide variety of other games, both in and out of Rosetta, and even lays out two separate 'what can/can't you play' sections to get down to specifics. Check out the full story if you're still biting your nails on deciding just how much you'll need to pony up to get your mobile Mac game on (also: stop biting your nails. It's a nasty habit).