If you are consider a X1950pro then you should definitely get the 7900GT. I was fortunate enough to have a friend who has a X1950pro so I can benchmark and compare it with my 7900GT, and at stock speeds the 7900GT came up marginally on top, but when it comes to overclockability the X1950pro was eating the 7900GT's dust. Also if you looked at the two cards you'd know which one to pick, the X1950pro requires a HUGE chunk of copper and a large fan for cooling while the 7900GT sports a puny heatsink and fan assembly.
7900GT is definitely the way to go.