One of the few games that dropping all the settings and staying at 1080p is not good enough on my 3870 and Athlon IIx4. I have to drop it to 1280x720.
What resolution do you want to play at? I also recommend a quad core. Phenom IIx4 945 at least. Then again, the Athlon IIx4 might be good enough too. I haven't tested my CPU with a higher end graphics card.
It is also important that you tell us your pricerange. I could recommend a lot of different things. Tell us what you consider smooth also. 60FPS? or are you good with 50FPS?