Not that good unless you already have them, as the cheaper 8800 GT is faster and much cheaper.
If you already have a GTS 640mb, well, I'd wait until they get cheaper to buy a second one. On average you'd see somewhere around a 25% speed increase.
how do you use both of them, do u just put them in a mobo that will hold both and not connect the 2 and wat do you have to do in your bios to use both of them
It will have two PCIe graphics card slots, and there will be a small SLI bridge connector to attach the two cards together once they're on the motherboard.
If it's an SLI mobo, you shouldn't need to mess with the BIOS. The Geforce drivers will take care of the rest.