This is a problem with all the Assassin's Creed games. They aren't very well optimized, and will only be able to maintain 60FPS outside of quieter areas, pretty much regardless of graphics hardware. On my 6870 I can get 60FPS easily in quieter areas, but once I hit more crowded areas, the framerate drops into the 40s. The game also stutters horribly with vsync on, also stutters a bit with vsync off, but not as bad as vsync on.
There isn't much you can do, all the AC games aren't well coded as Ubisoft isn't really interested in PC gaming. They believe everyone is going to pirate their stuff anyway, so why put in any effort? With the comments they've made, I wouldn't be surprised if AC: Revelations is their last PC game.