Deffo go for the single G80. SLI works best when benchmarking, other than that the real world gaming performance differences are not much, and are some times even worse than a single card. XMS and I both noticed this back on the 7 series cards.
G80 is wicked fast and has full DX10 support, definitely the way to go IMHO.
Asus Sabertooth 990FX | AMD Phenom II X6 1090T | 16gb G.Skill Ripjaws X DDR3 | Thermalright Silver Arrow
2 x Sapphire HD 6870 1gb CrossfireX | LG 10X Blu-Ray Rewriter | PCP&C Silencer 750 Quad | Fractal Design R3 Case
60Gb Patriot Pyro SATA III SSD | 1Tb WD Black | 3 x 2Tb WD Green | 2 x 360Gb Seagate Barracuda