Just to give you a hint of what I play and how it runs.
Alright, to start it off, I have the Black Macbook C2D with 1gb of ram. I can play Half Life 2, has great graphics, i can play it with full graphics on, its a little low on the fps side but i can deal with it, with medium settins on it runs fine. I have vista on it, I use aero, runs great. This laptop outruns my desktop, its a vaio, not the greatest desktop in the world but it gets the job done. I personally think the Macbook is a good choice for gaming if you dont need the greatest graphics and fastest ever gameplay. Its nice to know that you can take it with you and play it on your lap.
There are some games that run very nice on it, because they do not demand a very powerful computer, yet are very fun online games, like Day of Defeat, and Counterstrike. Im a big fan of Day of Defeat, and ive never had problems with it. There can be a downside to some games though. sometimes even if you set the graphics lower, it still runs slow, one game really comes to mind, Half Life 2 Lost coast. That is a very demanding game, most of my computers cannot play it without going frame by frame. I personally think the Macbook is great for gaming, just as long as the game doesnt go all out with graphics.
I think its a very fast, powerful laptop that gets the job done. If you are talking about some serious gaming, i would recommend what everyone else is, save up some more money and get a mbp. I was really close to getting one for college, but i had to pay some unexpected bills, i just ended up getting the mb. Goodluck with your choice, i hope this helps you out, feel free to email with any questions at ianisyourmaster@gmail.com
i am the kind of guy that loves to push my machines to the limit, i can also recommend some great software for your new little mac if you get it.
: )