If you have an SLi mobo then you have 2x 8x+ PCI-e slots, which is good for SoftTH.
All you'd need to do is get a cheap second GFX card (I find the 6600GT is good - you can get them for less than £15 second hand, they don't have turbo crap and are still useful on their own).
You can get a standalone bit of kit to do it, a Matrox TripleHead2Go, but it's £150. The "cheap option" is to get a second graphics card and do SoftTH.
Why don't you want to put a second card in that's different?
I would worry that the 7600GS lacks the power to drive 3x screens, though
(FWIW atm i'm using a 6600GT and an 8800GTX for SoftTH - detailed specs are linked to in my sig)