Hi there!
I also use a Mac (MacBook Pro 15") because I’m a video editor! I had the same question. Here’s my answer:
You can either download a nifty little program called Crossover Games for Mac OSX, which will allow you to play any game on Steam (including mods). It costs about 60$ or you can download the trial and find a serial crack online. However running this program slows your processor ALOT because it tricks the computer into thinking its running Mac OSX and Windows simultaneously.
OR you can dual-boot your Mac with Boot Camp if you have a copy of Windows lying around, but in order to do this you must have an Intel-Based Mac and I recommend you have either Leopard or Snow Leopard as Boot Camp comes packaged with it in the Applications folder.
The more recent models all have GREAT specs for gaming despite the popular belief that Macs suck for games (NOT true guys, just the Mac OS does
).
I myself could run HL2 on my Intel Core Duo 2006 MacBook Pro with everything on High and Episode Two with everything on High but shadows, HDR, and particles on Medium. Textures and Shaders could both be turned up to Very High. It looked great, although the Superportal was missing the Bloom Effect at the top so it looked kind of crappy.
So basically, if you have the money I’d recommend you buy Windows and using Boot Camp dual boot your Mac. Hope this helped!