I got this from another forum:
As you can see it only requires one PCI-Express slot. It also doesn't require SLI to run a single 9800GX2 card. He's confusing a multiple GPU card with two cards running in SLI mode. Its not the same thing.
To make things more confusing its often referred to as SLI because of its two GPU's. This is because it is as powerful as two cards running in SLI, not because it actually is SLI. You only need the SLI capability on the board if you are planning on running two 9800GX2 cards, which would be ridiculous.
The 9800GX2 maintains backwards compatibility with previous generation PCI Express 1.0 motherboards.
So, If you have a 650 to 750 watt power supply it will. And whatever you do, don't get a cheap power supply like those "Rocketfish" power supplies on eBay. They are really 550 watt power supplies that have been relabeled.
However, there is no point unless you also have a very nice CPU, like at least a Core 2 Duo E 6660 or Core 2 Quad Q 6660. If your CPU is slower than that or single core it will bottle neck your gaming performance.