You need to specify more on your system. What resolution, what CPU, how much RAM, what about your PSU?
Dragon age 2 is pretty solid graphics, and is no slouch. I recall I used to lower settings with a 680, when playing that game, but I also was after 70+ FPS, but with it on the highest settings, and 4x AA, I would get FPS down into the 40's at 1080p, if I recall. If you have AA cranked up to high, turn it to 4x, and you may need to lower settings.
Thanks. I'll try that. Yes, I am playing at 1080p have the i5 3.0 Ghz and 8 GB RAM. I have the Antec 550w PSU. I was just a bit confused why I have better FPS on AC 3. But since you have to lower your settings on the 680, I may have to adjust my AA to 4x and see.