I think SLI is overkill, with not that many benefits.
I would either do a 7900GT and volt mod it to 7900GTX speeds, or just go with one 7900GTX. Also keep in mind that you lose 256MB of memory if you do the 7900GT's.
Then again, I wouldnt buy either. I would rather spend my $ on a x1800xt 512MB or x1900xt, which I plan on doing in the next week or so.