The human eye can't distinguish between 30 and 60fps during gameplay(Although, everyone is diffrent.. Just like, to some, a 60Hz refresh rate will give you a migrane, while to others, 75 is fine, and yet to others, 85Hz is minimum) If a video card could run all parts of a game at a steady 30fps, no lower, It would be completely smooth. Consoles are programmed to run games at a steady FPS.
That said, "good" is considered 50-60fps in whatever resolution you're running at.
I play Quake3 at 800x600x16, Texture detail at the 3rd notch, geometric detail high, 32bit textures, with an average FPS of 42, a high FPS of 55, and a low FPS of 29. Very playable. K6-3/450, Creative Labs RivaTNT1, 128mb PC100.