If you already have a 7950 GT, why not get another for SLI?
The 7800GT is pretty much the same as a 7900GS which is basically an underclocked 7900GT. The 7950GT is by far the most powerful of those cards. from what I have seen, the x1800XT is slower than the 7800GT.
FYI, don't go by pipelines, there is so much more to cards than that. Look at reviews sites, they have what you need. I have found this chart invaluable when comparing cards:
just make sure you don't compare the cards via 3dmark, it lies like a rug.