The 7950GX2 is two 7950gt's stuck together. It's basically single slot SLI. Your x1900 is a little faster than 1 7950gt, but not a 7950GX2. Of course, this is assuming that the drivers for the GX2 are working properly, which is another story.
In any case your x1900 is plenty fast and you shouldn't "have" to upgrade for a while.