You CAN run more than one monitor with a SLI SETUP... but AFAIK, you can only use ONE monitor while USING SLI functionality.... In fact, Im pretty sure that with say.. 3 cards you could run 3 monitors. But the real downside is that if you can't get the SLI support on all three monitors (ie all three cards churning a collosal output on three monitors) you can "only" get windows display on them. Not sure if the future will bring a more scalable solution. The link provided says as much, the poster seems to think its a limitation with his drivers.
Though I have to say that I'm not happy with the performance increase on SLI myself (I got absolutely NO noticeable diff in BF2, the FPS might have gone up a few frames, but it was already high enough that I couldn't tell the difference)... So I just exchanged cards with my secondary PC, and now I have two cards (a 7900GTX and a radeon 9500) running in my primary PC. It's nice to have the dual monitor support for creepy shit where you have to read of one screen and type in another, and a LOT cheaper than buying a 40 inch monitor where both windows can fit
