I recently bought one, to tide me over 'till the new cards drop (alot) in price.
But I want to know what card that Nvidia has does it compare to?
Exactly which X1900 did you buy? The X1900GT, XT, XTX? Crossfire is ATI's dual GPU solution and you'd have to own two X1900s to use it. Two X1950XTXs are almost as fast as a stock 8800GTX in some cases.
Start at the URLs below and change the game and resolution to compare. For example
in Oblivion at 1920x1200 at high quality you'll find that the 8800 GTX gets 28.7 fps while the x1900 crossfire gets 27.1 (XT) or 27.4 (XTX).
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.