usually single card is better, as less heat is produced and power consumption is lower, and sli and crossfire do have their issues. The newer cards are less known for issues. Two crossfire/sli cards dont combine performance, they just split the load, and two 1gb cards stay at 1gb total, where higher end cards get 2-3gb vram. Lets say two 1gb 7770, cost on avg 125 ish, so total 250 and performance similar to 7950, which costs around $300, but u get again the power consumption and heat and restricted to 1gb of ram unless a 2gb 7770, but costs about the same.
So really the performance to crossfire/sli is similar to a higher end card, but most say if ur buying new get a single card unless getting high end cards for crossfire/sli
sli 660gtx non ti performs better than one 680 gtx, but uses around 75watts more power and more heat. which in that case is actually not bad for pricing, sli is about 440, and one 680 is about 450 low end