With SLI, both cards need to be the same model, and often from the same vendor, e.g. 2 EVGA 6600GT cards. To be honest, I wouldn’t go for SLI right now, since you’ll need another high end card which will be very expensive. You may want to wait a while, and buy it for SLI when the card’s cheaper.
ATi’s Crossfire allows two different models to be run.