You can dual boot your Macbook with Windows, but if you play a few source games and some Guild Wars, is it really worth the hassle?
Steam will run natively on Mac OSX now, so you can just wait for them to get stuff like DOD:S moved over. I know Altitude is supported as of now, as well as several other games.
But honestly, if you see yourself playing lots of games, don't buy a Mac, or if you must have one, take your desktop to college. I have a MacBook Pro and I don't ever use it to run games, just notes and a few videos here and there, and I mainly just game on my desktop.
BTW, Apple doesn't know what thermal grease/paste is, so if you start overheating a bunch, you may have to crack the thing open and reapply. Apple uses 3g of thermal paste per heatsink contact area (it has 3) and basically just suffocates your cpu and gpu.