aaah it depends on morality you see, 7900GT is modded with removable conductive ink, remove it if you card dies and RMA it.
the voltage increase you need to get to GTX speeds is 0.2 -> 1.4v the same as a GTX so longevity and temps are not a problem.
i would only advocate the 7900GT if you want to save £60 and are into modding things
if you have the money to burn on a 1900XT then go for it.
the 7950GT is purely an overclocked 7900GT, i wouldnt bother.
FWIW my 7900GT does 650 core and 1800 mem. that is GTX speed