I was able to get 2 8600 GTS SLI cards for less then the cost of 1 8800.
I have not had a chance to "tinker" with the 8800 so i can not speak to how well it performs, however my two 8600 cards running in SLI are just amazing. I stay at a constant 60 FPS even when in major cities with all the graphics settings maxed out and the highest resolution.
There can be issues when trying to run dual monitors with SLI, but you can always disable the SLI feature when multi boxing.
If you are unsure of the 8800 series, I would really consider the 8600 either solo or in SLI.
Just my 2 cents worth.
