The game's framerates vary wildly depending on where you are in the game. In places like the Frontier or the Homestead, 60FPS is easily achievable. When you get into crowded urban areas like Boston, your framerates take a dive when the screen gets very busy, eg. lots of NPCs.
AC3's performance problems aren't really GPU related, the game is very heavily CPU bound, it isn't well threaded. The game rides one core very heavily, and doesn't use the other cores very effectively. If you have an overclocked Sandy Bridge or Ivy Bridge CPU, you'll probably get good performance.
I get okay performance with my first gen i5 if I turn the environment details down from very high to high, as something in the environment details setting takes up lots of CPU power, and causes my framerates to dip below 30 FPS in Boston. When I'm in Boston, my framerates will usually be in the 30s or 40s, with my GPU usage capping out at 85%.
TL, DR version: Your CPU is more important than your GPU when it comes to performance in AC3.