The best bet is to look at your motherboard yourself and look for the slot that your video card is plugged into.. either AGP or PCI-Express x16. If its PCI-Express x16, and there is another slot just like it than it is SLI.
As for another card, you used to have the same model card for SLI.. but I saw a driver that allows for different model cards to work together with SLI. The new card is up to you.. depending on your budget. I'd get a 7800GT.