at that resolution, the X1900 will help a lot, but I still think that plopping another GT, will be cheaper and a wiser choice. Then again, think of G71... its just around the corner.
A thing I suggest you do is get an EVGA 7800 GT, and put it in SLI with your current one, and then step up to the 7900GTX by adding some bucks throu their step up program. Then you can still sell your current GT, but you wont have to wait until march to enjoy good frames.
If you go with ATI you will have a fast video card now, but for sure not as fast as the 7900 GTX.
my suggestion will cost 300 for a GT + ad 300 for a 7900 GTX (estimate of 600 MSRP) - 200 for your old GT = 400 (but hell you will have the fastest card on the planet in march).
consider a full upgrade thou, that processor is leaving you behind...