Well - The OP has an Intel chipset, which means Crossfire/ATI. As pointed out, there is software available on the web which would allow SLI on a Crossfire board. But to my knowledge it's not officially supported by anyone, buggy to begin with, and in a dual GPU world you will almost always need patches and/or software updates for each game you want to play.
So the 'supportable' answer to the question is "No, you can't run SLI because you don't have an nVidia chipset. But you *can* run Crossfire, which is ATI/Intel's way to accomplish the same thing.
Speaking of mainstream technology as it stands *right* now, know that because of current PCI bus limitations two video cards are most assuredly *not* twice as fast as one. Why? You get one X16 lane and one X4 lane, at best, and are FAR more likely to have X8 twice. You would need a brand new - and only available as of last week~ish - be well advised to wait until the BIOSs in these have matured some - Motherboard sporting an X38 chipset to get X16 times 2. That chipset only supports Crossfire - no SLI. Also - While you can have DDR2, the X38 chipset is optimized to run DDR3 Ram. Which is freaking expensive and only demonstrably better than DDR2 at very high clock rates. There are articles on this site covering this stuff should you care to delve deeper.
It's true that when splitting hairs at the highest level, nVidia's current cards are superior to ATI. But to be brutally honest, the difference is measured in 10's of Frames Per Second, at FPS rates which are already far higher than the human eye can ever hope to differentiate. Given the position you are in now, IMHO, your best value per dollar spent would be to buy a decent (not best) pair of 2900's, setting them up in Crossfire on the Mobo you have now, and calling it a done deal. If you really want SLI, you'll need to spring for a new Motherboard, as well as the pair of video cards.
But having said all that: Personally, I still think that a single, superior, GPU remains the best bet if price/performance means anything. And that means an nVidia 8800GTX or Ultra as of this writing. In My Very Humble, Mildly Cynical, and Jaded OPINION, the best use for Crossfire/SLI is to extend the life of the card you spent $500 on a couple years ago by pairing it with a twin for $250. You'll save yourself $250 towards your next build, since by the time the dual setup runs out of steam, you'll be at 4~6 years and will have to gut your case and rebuild from the bottom up anyhow. But that's just my $0.02, and you should feel perfectly free to ignore me on that.