Valve games are notoriously difficult to run through Wine (what Crossover essentially is). The game may run, but it'll likely be pretty unstable. Not only that, but none of the Steam features will work (Steam friends, community stuff, etc).

If you have Vista installed, you're much better off just booting into that when you want to play games.

So why on earth did you get a Mac if you wanted to play Portal and HL2:EP2? You'd save a ton of money by just building a PC with good quality parts and installing Vista on it.