Dont listen to Ripley, comparing a 2009 cpu to a 2012 one is highly unfair. Otherwise an 3.8GHz, the phenom knows 80% as much as the i5-3570 at 3.4, the difference is not as drastical, so no worries.
CUDA/PhisX has nothing to do with stutters, especially not in AH.
Let me guess: when you start to play, you get a decent fps, but after a while its decreasing to the 10-20 range? I have the same problem with a 5830, Fester had the same problem with a 560ti, guessing it is a bug with the V-RAM. Alt+tab in and out usually solves the problem for me.