In most games, an X1900XTX CrossFire setup would shred a single 7950 GX2. Remember that it actually uses a pair of mobile graphics setups; hence, performance is only like that of a pair of 7900GTs, not GTXs.
In Oblivion, and possibly F.E.A.R. as well, a SINGLE X1900XTX could beat a 7950GX2, as due to the X1900's inordinate number of shaders, a single one has better shader power than a GX2, which has a pair of mobile G71 cores.
A 7950 GX2 will beat the x1900xtx once you get to a resolution of about 1600x1200 in most games (Oblivion and Fear Included). You simply can't run two 7950 GX2s because nVidia won't provide the drivers for end consumers. I think the gx2 has a slight advantage over other sli/crossfire solutions because the communication between the two gpus is a bit tighter. I beleive this shows up in Black and White 2.
One disadvantage a gx2 or any other crossfire/sli solution may have is frame latency. Because most configurations prefer the the gpus to alternate frames you end up with a triple buffering situation. Which means the time between when a frame starts to draw and when you actually see it could be double that of a single GPU solution. I assume this isn't the case in a split frame (scissor) setup. It's also a good reason to avoid a quad sli