I read the link that jeff_2087 sent.
I am of the opinion to get the 8800GTX. Even though 2 8800 GTS's are faster, I'd save difference in cost and then when the 8900GTX comes out, you can sell the 8800GTX and take the money saved and get the 8900GTX (or whatever the next latest & greatest model will be called).
The other option would be to buy another 8800GTX later down the road. I would always buy the most expensive single-card solution you can afford and then just SLI it down the road.
If you buy 2 8800GTS, you can't do an SLI upgrade without buying 2 new cards.