Actually, considering the settings the 7800GTX can do that the x850 will not be able to reach, I'd say the GTX's image quality will be better at those higher resolutions + AA/AF settings.
However, as was mentioned, you're really comparing the wrong generations. The direct competitor to the GTX is the x1800XT, which would more than able to keep up with the GTX and has the nod in IQ.