Well I ran 3D Mark benchmark: Firestorm (for high performance gaming PCs) on my EVGA GTX650 1GB and scored a mediocre 1733, maxed out about 12fps graphics test, 22fps on physics test, and a measly 5fps on the combined test. But it does run Fallout:NV on ultra settings flawlessly. I'm awaiting my GTX660 FTW 2GB in the mail, when I get it I'm going to run the Extreme Firestorm bench and hopefully it scores much better.
And as far as SLI goes, I've never had a multi-GPU system but I've always heard that a great single GPU usually blows any multi-GPU set-up of any lesser spec individually out of the water. But I'm sure you'll upgrade your mobo eventually, most all gaming style boards have at least 2 x16 slots, mine has 3 and only using 1 so far. I feel like I'm wasting potential but we'll both get there eventually hopefully after less games have so many bugs with SLI.