Read the sticky on dual graphics cards. Unless you plan on spending some serious money on graphics cards that can run high resolutions, the results of this article show not much improvement over a single card. Considering the price difference between single and double, I'd just sink some money in a nice 512mb card. After all, you do have a pretty powerful CPU, so ther is no problem there. If you do decide to go SLI, you will definantly have to get a new power supply. Beefcake is right, 1GB just won't cut it if you upgrade your graphics card. I'd definantly got with at least 1.5GB, but 2GB if your wallet can bare.
~Justin