You have to run two like cards. So you could do two 650s or two 660s. But honestly if you are going to spend that much money you should just get a single GTX670. You'd then even be able to scale back a little on the power because it is one card. Crossfire/SLI only really makes sense if you already have one card and you get a good deal on the second or you are exceeding the capabilities of the best single card.