One subjective thing is what is "ok" for everyone. What is your minimum standard you want before you start thinking about needing an upgrade? After you put on Ultra and throw on some AA even high end cards take a beating.

1920x1080, high or better settings, x2/x4 AA, 40+ FPS. Is what I hope to maintain over the next few years for newer games. Once I get below medium settings getting less than 40 FPS it may be time to upgrade again.

I do have a HD 68xx in my old PC now instead of a HD 48xx. Of course depending on the game I saw some nice boosts for the little before/after testing I did. Then games like WoW or SC2 you saw more boost from a better CPU than the E8500.