60+ is good, anything higher than that you won't notice. Anything lower than 30 is considered unacceptable as you really feel the slow down with any fast games (FPS, etc). Some slower games, like Oblivion (RPG), can still be played fine as low as 20. A TBS may be able to go lower just because the action isn't as important as the moves you make, but that should be enough information for you.
But also keep when you're reading review sites that most reviewers (when comparing cards) will turn up the eye candy to maximum, enable 8x Anti-Aliasing, 1600x1200 resolutions, and so on which can have a HUGE impact on frames per second.
Playing FEAR at 1024x768 with no AA will get you MUCH more FPS than playing FEAR at 1600x1200 with 8xAA...
With RTS games, 20 fps will look great. Source (Sin, Counter Strike, HL2) games look good at anything above 40 fps. FarCry looks really well above 60fps.
Generally, you won't notice any slow down as long as the frames are above 30 per second.
RTS and RPGs don't need high FPS while with First Person Shooters, you need higher frames per second.
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