From some forums ive read about SoftTH, you dont have to have the first two monitors on the same card and the third on the second, you can have the primary on one monitor and the other two on the second card. which leaves it to believe taht you could run SLI and push the first monitor and then use your second card to push the others...now currently using SoftTH with an 8800GT and a nvidia quaddro, I would recommend getting a decent second card to push the other monitors. Maybe like an 8600GTS or something...you can pick those up now a days for 60 bucks...
For me having a quadcore and 2 8800 GT's, the only game that requires SLI to run smootly is Crysis. So running the three monitors off my two 8800GT's gives me the best possible performance imho, as I can run Call of Duty 4 at 3840x1024 with max details and the nVidia control panel set to Max Quality.
I have the center monitor on the first 8800GT as it's drawing 32bit color, and the left and right on the second 8800GT as it's drawing 16bit (YUV12) color.
Once you enable SLI in XP, you cannot use any other video display output from an nVidia card, SLI forces output from only one video connector. I tried with a PCI nVidia 6200 and it with it installed, I could not even enable SLI.
I can get a very old ATI Rage 128 pci working (as it uses a different driver) but it would not be good for SofTH driving the left and right monitors.
Supposedly in Vista, you can get SLI 8800's and a pci (or pci-e) nVidia card working, but I tried it again with the 6200 and couldn't get it to work.
For now, very very happy with the 8800's in non-SLI mode with SoftTH...may get a TripleHead2Go down the line, but then again, with SoftTH I can enable larger than 3840x1024 ( 3 x 1680 x 1050) on a few titles and you can't do that with a TH2G (yet).