techfuzz, I posted a similar thread a few months back, and the conventional wisdom at the time was to keep my 7900 GT KO.
Typically, from what many have posted and my own experience, the time to upgrade is when your card can no longer handle the latest and greatest games with moderate eye candy enabled.
I am able to play Oblivion, Supreme Commander, Company of Heroes and others without taking a framerate hit...on a 22" LCD at native resolution.
I know I can probably get away with my 7900 GT KO at least until Christmas...at which point the 8800GTS 640MB will probably fall rather nicely into both of our budgets.