I would personally be suprised if AC3 is very cpu intensive. I could be wrong though. RAM may kill you though, it's a 40$ updgrade to get 8 extra gigs, so it's pretty easy to fix.
You may have to reduce a few features from where you want to be, such as turning AA down, dropping shadows a bit, but I would expect you could find a smooth and good looking setup (assuming 1080p, 720 would likely not pose much trouble)