There is a site with the numbers out already. A pair of 7800GTX stock in SLI will get ~13k in 3DMark05. A lone 7800GTX will get about 7700. A 7800 Ultra will get about 9000.
Right now, a pair of 6800 Ultra stock in SLI gets ~10k in 3DMark05. People estimate a pair of X850 XT PE in CrossFire will get about 11k (given that individually, X850 XT PE score about 6100).
I personally like ATI's CrossFire solution better than SLI, simply because each game doesn't need it's own driver support and profile. CrossFire will also let you mix and match video cards. Right now, I can buy an X850 XT PE for about $400, and the cheapest 7800GTX will be about $150 more.