I would personally go with the x1900XTX, since in most of the games I play it performs better than the 7900GTX, and the use of HDR+FSAA is also a plus. AVIVO makes movies look awesome as well.
Well I would still go with the XT, since the only difference is a slight clock/memory speed decrease (which can be easily reached by overclocking). Not to mention that the x1900XT is slightly cheaper.