I use a venerable and otherwise unused XP32 for my Boot Camp needs, when I can be bothered with it.
Crossover running Source slow has everything to do with Crossover not using 3D acceleration in most configs.
You can hack around a bit and enable proper 3D in Crossover, or you can compile/run vanilla Wine, both will run Steam/Source nicely.
Steam is actually a godsend for multi-OS peoples. Several games that I have will run fine via Steam (no CD checks), but will fail when using the original media. Using cracked executables greatly increases the chances of something breaking in a difficult-to-reproduce manner.
Behemoth - media server
Galactica - test lab server
Falling Outside the Normal Moral Constraints - MBP 15
Donnager - M4800 DTR
Rocinante - E5530 testing machine
Saratoga, Ticonderoga, BunkerHill - the Hydra desktop PC-thing.