You do need 3 video cards (2 for SLI, 1 for the other 2 monitors).
Here's a forum post that has links to the places to help you out. It sounds like nVidia will release some driver support for SLI and multiple monitors, which will drop the number of GPUs to 2.
Here's the link for Vista:http://www.overclock.net/nvidia/31
XP is easier, since you can load ATI and nVidia drivers together.