The must be of the same chipset, in general. You can technically use an Nvidia and AMD card, although it is highly recommended that you avoid that route as hybrid drivers are not the best, and are not frequently updated like official drivers.
For SLI(Nvidia cards only) they have to be the same model of GPU. With Crossfire(AMD only) they just have to be the same family, for instance a 7970 can crossfire with a 7950, but not with with a 7850.
For SLI(Nvidia cards only) they have to be the same model of GPU. With Crossfire(AMD only) they just have to be the same family, for instance a 7970 can crossfire with a 7950, but not with with a 7850.
Can I combine at least GTX 660 and GTX 660 ti? Or GTX 660 with GTX 660 twin frozr edition?