1900XT/XTX
Faster in majority of games by a certain margin
Better Image Quality
HDR+AA ability which lacks in 7900GTX
Better AF
Will perform better in the upcoming shader intensive games as it has more shaders
7900GTX
Faster in some games and in those it is the clear king.
Faster than the radeon in many paper benchmarks but when the image quality is chosen so that iamge quality is same there the 7900GTX looses. 7900GTX can give more performance at lower image quality settings.
Better AA although you can't use that for new games as it isn't that fast without SLi especially
CONCLUSION
Unless you are mad about playing a few games only like Serious Sam 2,Pacific Fighters etc where the 7900GTX might be the clear winner or for that matter even the 7800GTX, by and large my choice is as follows if you forget about the money point of view, just performance and overall section criteria for me and all,
1900XTX OC>1900XTX>1900XT>7900GTX OC>7900GTX
1900XT box overclocked = 1900XTX and is almost the same in most games with exceptions, 1900XT can be overclocked to perform equivalent of a 1900XTX provided the card can be overclocked as much and you have the right cooling.
Price vise, in US, 1900XTX and 7900GTX are at par and in India 1900XT and 7900GTX are at par.
Plain and simple, get a 1900XT around 30k, PM me for details.
7950GX2 = underclocked 7900GTX SLI
it performs between 7900GT SLi and 7900GTX SLi
Also, keep in mind that in terms of minimum fps 1900XTX will/should beat even 7950GX2 in Oblivion. But overall 7950GX2 is a better performer, but keep in mind that:
1900XT is cheaper
better image quality(IQ)
better AF
HDR+AA ability absolutely requried in games like Serious Sam2
although nvidia has better AA, there is no point when you can't even use AA and I for one don't want to compromise on HDR, however there are others who do.
with future games the peformance diff between 1900xtx and 7950GX2 may narrow as 1900xt is good in shader extensive games,
7950gx2 : >35k
1900xt: around 30k