The GTS makes alot more sense, and the performance benifits will be more global, unlike SLi. Also less hassle.
There are areas where the GTS would lose out to even a single card, but they are rare, and for the money the GTS is a better investment than any SLi combination. However it might be worth waiting a week for the initial price to settle at some store to closer to MSRP, but compared to SLi'ing some GF7950s, it's cheaper and IMO better.
EDIT: As for the MoBo, as long as it's not too expensive the SLi MoBos usually have nice extras, and it leaves the door open for you to either SLi the GTS or to potentially do a physics solution add-in later. Of course it depends alot on the price. But for MoBo technology for intel you should be OK for now for a while, whereas AMD will change a bit sooner, but likely won't matter much either.