DO NOT GET THIS! It isn't as fast a a 6800GS, uses more juice, has 1/2 the memory, and costs $20 more than a GS and that's before MIR
New Egg
Yes this card says it has 256MB of RAM, but it's split between the GPU's, 128MB per chip, making the total amount of memory available for textures, and what not just 128MB, unlike, the cheaper, and better performing 6800GS that can address a full 256MB of RAM.
As for 2 of these in SLI that's just plain stupid, even if you could do it, it would cost you $400 total for a setup like that, and it would perform worse than a 7800GT, that only costs ~$285. You would get worse performance, and not get newer features like transparency AA.
I hate to thread crap but this is not a hot deal, cancel your order if you can, save some dough, gain a few FPS's, preemptively avoid some SLI incompatibility headaches, and get a 6800GS