Yes, with hardware and software it is possible. However, no one has two yet. I've probably been the most vocal on this subject, and had you done a search, you would have found two threads by me on the feasibility of this setup. Even Google has my threads on the first page.
I, for one, will be doing this. I cannot pass this up.
You cannot, as of yet, SLI two 8800 GT. When the GeForce 8 drivers get hacked for this to happen, it will only be supported in Windows.