Agreed with Psycho, only an HD5750 and HD5770 are crossfire-able.
However, an HD5770 crossfire is very potent, especially if you overclock both of them.
Didn't think to ask in my initial post, but what is the best way to OC a video card? OCing a CPU is easy enough to understand, though I've never tried it myself, but I'm not familiar with OCing a video card. Thanks.