SLi will not give you a huge performance boost over the GTX but there is some increase but the two cards are more expensive. I would go with the GTX. However........the M80 will be out in the not to distant future and you may want to buy just a 7900GT. I have a BFG 7900GT that oc's without any difference in core temps at 550/1600!! It will do any game at the highest settings. Save your money for the next generation DX10 complaint cards.
Noli Nothis Permittere Te Terere!!
EVGA 680i, Dual EVGA 8800GT'S (650/950), 4 gigs OCZ DDR2 1066, Intel Q9450 at 3.8 Ghz, Koolance Water Cooled, Dual Plextor16x DL DVD+/-RW. Dual WD 250gb 16mb Cache Sata 3.0. 3DMark06 score = 19,168.