I have the mid-2007 MacBook. 2.16GHz, 2.5GB of RAM, 120GB HDD, "SuperDrive". OS X is a great OS. It is lacking in some things and while it doesn't have Windows problems, it has its own set of issues. You'll find DVD playback lacking, if thats an issue at all to you.
If you're thinking about a gaming machine, that means you're into hardware too. You'll be satisfied with the processor speed in the MacBook. But you'll be very disappointed with the GPU. The X3100 doesn't even perform as good as the GMA 950 in nearly all real world situations, even with updated drivers. Both GPUs perform about as good as the low end GeForce 2 series from 8 years ago.
You'll also find that OS X doesn't take advantage of video hardware at all, even if you have an iMac or MBP with a GPU capable of hardware decoding and such. As a result, watching any any video will take an unnecessarily high amount of CPU time. Where DVDs will only eat up about 2-3% CPU on Windows, on OS X you're looking at roughly 25%. High def H.264? You can expect to give up a single core for that in OS X while the CPU in Windows would sit back and relax while the GPU does all the work without breaking a sweat.
Another thing to remember is that the iMac uses MOBILE parts, except for the hard drive. The CPU, GPU, and chipset are all laptop parts. So don't buy it if you want to game at all.
Basically, you're buying a Mac for Mac OS X. You're NOT buying it for the hardware. If you want to play games, you're better off building an awesome gaming PC and then getting a Mac mini as a secondary system to play with OS X.