You asked which is faster, the 7950gx2 is faster of the two. By a good margin on some games, a smaller one on others.
X1950 XTX is what I think you should get. Speed isn't the only factor, argueably at this level of high end performance it is the least important thing to consider.
Honestly at the (lower) resolution you'll be playing at I'd recomend the x1950. It costs less, it will play everything out there for a long time to come, I feel AF looks nicer on the ATI card (it's going on a very large screen, you will appreciate IQ more) and stuff like HDR with AA works on ATI -it doesn't on the Nvidia, and at lower resolutions you really want AA, even more so if you're blowing this up on a huge projection screen, this would only be a problem in stuff like Oblivion and a handfull of others, but personally it would be a big consideration for me.
Add to that you have a Crossfire motherboard.
x1950 XTX in Crossfire is faster than a single 7950GX2 in about 99% of games from the stuff I've seen meaning in a few months or even a year you could add a master card and get a nice jump if you wanted more performance.
And yes, I own the 7950GX2, it's a great card, but I think the ATI will give you the same frames but better quality for less money.
