Crossfire technology has come a long way over the las couple of years. You can expect to get an 80% improvement in most games. I have two HD5850s, but only one in each of my PCs. I was lucky enough to get them when they first came out for $259 each, and they are head and shoulders above my old 8800GT (anywhere from 2x to 3x faster). Plus I was able to overclock each card by about 25%. The new ATI series of cards are really nice.

LFR1100 Actives,QS10HPx2,QS8x2,EP800,M3x4,M3x2 (Wood),M5HPx2 (Wood),AxiomAir,ADA1500-8,ADA1500-7