30 FPS is generally considered the industry minimum for enjoyable games, but 60 gives a smoother performance and if you don't really notice whether it's 60 or 80, 60 seems about right for you.
(A quick comparison: http://www.youtube.com/watch?v=9yBsgtA8WSE)
It just depends on whether you value image quality over response times: if you're playing a lot of multiplayer, you'll want to keep those frame rates high in order for a more responsive game and better performance. If you're casually chilling in a sandbox, maybe dropping to 30 for some nicer buildings might be worth it. Your call.
I'd concentrate on minimum FPS rather than mean average. i.e. try to make sure your FPS doesn't drop below 30. Even if your average is 60 FPS there may be some points that are really heavy where it drops significantly and this is where gameplay can be really affected. Like Bemused_Fred said, it also depends on what sort of game you're playing.
Ew. 45 FPS shouldn't even be considered playable in any competitive multiplayer game :x. Perhaps in single player if your hardware is forcing you to compromise, but never otherwise. Visibly degraded vs. 60 fps.