I'm playing on 1920 x 1080. I am getting good fps, but when I am standing on the highest point of Whiterun and I am looking over Whiterun, the fps drop to 40 or below. I've also tested it on Assassin's Creed Revelations and I am having the same problem when looking over the city from the rooftops. I am getting good fps in Saints Row 3, but again when I am looking over the city or when there is a lot of objects in my field of view, then the fps drop to around 40. Battlefield 3 - no problem, getting 60 fps +.
Your CPU is holding the performance back(most noticeable in AC/SR3 clouded places)
So upgrading it would give you a notable boost in performance but if you can't upgrade,there is nothing to worry about. 30+ FPS is easily playable
I tend to agree with Maziar.I don't have Assassin's Creed so i can not test it.I have BF3 i never dip that low lowest it goes on me is around 50fps with a single MSI 670 PE 1920 x 1080 so it's your cpu imo.