Here let dig up some more information on Crossfire vs SLI.
Crossfire offers more display modes then SLI: SuperAA, AFR, Supertile, and Scissor.
Crossfire also works with some chipsets already, as oppossed to nVidia which makes you buy a nForce 4 SLI chipset.
Also, with the new CE (Crossfire Edition) cards, you can hook up FIVE monitors. I don't know if nVidia can do that or not so I'm not sure if that's better or worse...
This link shows how much better a X850XT CE performs to that of a 6800Ultra w/ SLI.
There wasn't any specs on the Crossfire X850XT vs the 7800GTX SLI.
That site also said that the R520 chip coupled with Crossfire should be able to outperform the with 7800GTX in SLI.
So....I still say go with Crossfire technology and get two X850XT PEs.