Crossfire does scale better than SLI does.
Single card vs single card, the 8800 GT wins.
but, crossfired 3870's will perform about on par with SLI'd 8800 GT's
I have an 8800 GT coming my way.
Though considering the board I want supports crossfire, I might even consider getting a crossfire setup.
the good thing about the new cards (G92 and RV670) is that they consume much less power than the 8800 GTS I currently have.
I believe my power supply will be able to easily handle two of either cards.
*edit*
come to think of it, if I do go ahead with that, I'll only need to get new RAM and a power supply, then I can have two complete high-end systems.