Drawing from personal experience.. I will avoid ALL 7900 series from Nvidia.. actually wincing if I see the number "7900".
However, I've also heard that the X1900 series before the XT had problems w/ heat, which was improved w/ the release of the x1900XTs...
The problem w/ the x1900 cards before the xt was that the exhaust is blowing hot air in the casing, w/ the XT series, the exhaust is actually at the back of the PC, blowing out.
If I'm to ignore the defects from the 7900 (unlikely), and do a comparison, then:
-7900 card is a less noisy than the x1900
-performance wise, the 7900 seem to have the edge (if it's not malfunctioning and not sure about the Nvidia GS vs ATI GT), however absolutely no HDR + AA for 7900
edit: my views are from 7900GT-TOP & X1900XT tho