I have the 7600GT, and runs everything pretty well maxed out at 1024x768 and 1280x1024 resolutions. The 7900GT can game at even higher resolutions with hardly any slowdowns at all.
It really depends on your monitor and if you can game nicely at high resolutions with AA+AF on. There's no point in spending so much money now on one when you're stuck with a 1280x1024 LCD monitor. Now, if you got a nice CRT monitor or a LCD with good resolutions, then the 7900GT is a good bet.