Hey, man, what if he got a HydraLogix board? He could totally rock the two together. Joking aside, CrossFire isn't supported cross series(HD5xxx and HD6xxx, for example).
That's what I was referring to.
Do you find HydraLogix funny? It can pair two of ANY graphics cards so long as one of them has an NF200 chip or the motherboard has an NF200 chip. Heck, you could pair a GTX 295 and a Radeon 6970 or some other wacky combination that way if you wanted to. The motherboard link that I gave has one such chip.