You are definitely on the right track.

While it is possible to run SLI on two monitors now (very recent development) I would recommend against it. Running 10 instances, I would want 4 screens: 1 fullscreen display, 2 splitscreen displays and a web browser display. A single 4870 X2 would do the trick, or a pair of 9800gtx's -> 260gtx's ->280gtx's would do the trick (in ascending order of cost and performance)

If you are not considering any more than two monitors, I would consider the 4870 or the 260/280 only.