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.