I can safely say that I wuv my 7800 GTX
Maybe SLI'd 6800 GT/Ultra can out bench a single 7800 GTX (as can SLI'd 6600 GT beat a single 6800 GT, mind), but as of now, SLI'd 7800 GTX's are the ultimate desktop GPU setup, no question.
I'll be adding a second most probably later on in the year, unless the R520/X900 really, REALLY
outperforms it significantly. Or I'll get the '32 pipelined 512MB Ultra Extreme OC' version of the 7800, or whatever, that nVidia no doubt release in a grudge match with ATI