Wow, there's a wealth of misinformation in this thread.
I would also recommend switching the PSU. For SLI, Nvidia's specs on the 8800GTX card power requirements is 30A on the +12v rail (doesn't matter how many rails), and 4 PCI-e power plugs (2 for each card). I wouldn't worry about the wattage as long as it's a good brand and those specs are met.
Seasonic is well known to have quality components and ALSO be quiet at the same time. Especially the M12 series.
If you want Nvidia for graphics and SLI, then you need to look at the 590SLI or 680i chipsets. For ATI and Crossfire, it would be 975X.
Frankly, SLI is useless for 8800s right now anyway (driver support is horrific). It's a nice option to have, but I really question the sanity in it.
By the time I was ready to replace my 6800GT with a higher performing solution, I found that it was more cost effective to just get the better card instead of going SLI. One 7900GT was WAY faster than a pair of 6800GTs, and 2 6800 GTs wasn't going to get me the new shaders etc that were out in the 7000 series.
Now that I have the 7000 series card, getting another for $200 to get more performance (not double because scaling isn't perfect) seems silly when I could wait a couple months and get ONE DX10 enabled card that will outperform a pair of 7900s overclocked. Plus, I can get $100 or more for my current card by selling it now, and that's only going to drop as time goes by. Take $150 off of the price of a GTS, and you're within spitting distance of the price of another 7900GT. PLUS, you'd get DX10 goodness.
SLI has not yet seemed worth it to me, even though I shelled a good $50 on my mobo when I bought it to get one that could handle it. It would have cost even more to get a mobo that OFFICIALLY did SLI. Heh.
Anyway, food for thought.
OK so it's obvious now you've EDITED your OP (I hate that), so all my advice is junk.