Normally you would only do SLI Xfire (SLI is nVidia's "version" of xfire) if using two older cards, or if you REALLY need the extra bump. You should only use same cards (brand is irrelevant) and the performance is like this:
1 card, 60fps. (i.e.)
2 cards xfire, you'd get 90-105fps. ALMOST but not quite x2 the performance.