First of all, for all intents and purposes, Macs ARE PC's, and have been since 2006. The only real difference between them is the OS that comes pre-installed on them (which in my opinion, and in the opinion of most of the other people on this forum I'm sure, is far superior to Windows), and the build quality is higher than the average PC is. As a result, any Mac will be able to game just as well as any equivalently specced PC once Windows is installed on it, it's as simple as that.
As for the MacBook and gaming, it really depends. If you're looking to play games with settings cranked all the time, then no, obviously the MacBook won't be a good gaming experience if that's your benchmark. That said, now that it actually has an nVIDIA graphics chip it can at least run them at a playable level, which is currently more than can be said for most other comparably priced laptops with the same feature set and footprint. So if you're planning on buying a laptop that works great as a general purpose work machine but still be able to want to game in your spare time and don't mind turning down the settings, the MacBook does a fine job. If you need to be able to play every game maxed out, and that's your top priority, and you don't mind throwing portability and battery life out the window, get a higher end PC laptop designed specifically for gaming, or better yet, just get a desktop since you'll get more bang for your buck, not to mention more upgradability.
Personally, I'm content with having a compact laptop that runs OS X for work, but also has the ability to run games when I need it to, and then just go home to my desktop and game consoles to do serious gaming when I'm not working. So, I went with the MacBook.