performance is comparable; each would have their share of slight wins depending on game (oblivion seems to be the exception, favoring ati in outdoor areas where fps is most important).
you will get substantially better texture filtering with that x1900; slightly better AA on alpha textures with the 7900. having had both i favor the overall image quality of the ATI by a fair margin.
if power consumption and noise are a concern, power would favor nvidia by a fair margin, and i would speculate noise would favor the 7900 as well (i only speculate as i haven't heard the fan on the GTO, but the GT fan was rather noisy). nvidia also has a smaller profile, which may be important if you have little room in your case.
if you plan on going dual cards at a later date, SLI is the more mature solution between the two, however both SLI and Crossfire work decently, but SLI is the more 'elegant' solution (editable profiles, no dongle, no master/slave).
i stuck with the ATI as image quality and performance are the major factors (after all we spend 100% of the use of a video card staring at the screen), while the points that favor nvidia are less important to me personally.
if you're not one who really scrutinizes such things (an example would be my wife, who thinks WoW looks better on my system, but she's perfectly happy with how WoW looks on hers (7800GS), even when running in (ugh) performance mode. heh.. to each their own, I guess), you'd likely be happy with either.