I have a evga 7900GT in my computer right now. If I wanted to go for an SLi configuration, am I limited to a 7900GT. Could I use a 7900GS What about a 7950GT?

I know that you can't use the 7950 with your 7900GT, but the 7900GS MIGHT work... It's not supposed to, but if you underclocked the GT there might be a way to get away with it. Without having tried it, I can't tell you. However, underclocking your GT would be a high price to pay to put it in SLI with a GS.