oh, agp, so you would need a new gfx card, and you would need two IDENTICAL cards = pricy
Google says they do make mobo's with socket 478 with SLi.
I used to have SLI (2x 6600GT's), afaik when SLi is enabled you can only have 2 monitors, for me SLi caused a LOT of instability, so i disabled it and messed around with a few spare CRT's, ran with 2 CRT's, but i found that it only likes to run a 3D app across two monitors from the same card, when you start running an app across two graphics cards (which you have to do for more than 2 montors) the FPS DIES.
EDIT - the same happens with two monitors across 2 cards - i would've spent more time messing about with out, but i was pissed off with having a £60 paperweight sitting in my second slot so i got shot of it and traded up to my 7950GT