The main difference is the lack of L3 cache in the Athlon II chips. There wouldn't be a massive difference but there would be one particularly when gaming. Another thing you have to consider is if you go for a Black Edition of the Phenom II chip then you can also overclock easier which will also help. I've posted some benchmarks comparing an AthlonII and Phenom II chip of the same clock speed so you can judge for yourself. Realistically but an Athlon II would probably be more than enough but some users want the absolute max. It's all down to what you want out of it. Of course, you also have the option of overclocking the Athlon II as well to bridge any would-be gap.http://www.anandtech.com/bench/Product/105?vs=81
Hope this helps!