You did'nt mention your requirements fully, as to what you'll be using the PC for. If it's just for gaming, really a quad should be plenty.
Actually, to be honest most games don't even require 4 cores. So a high end dual core is plenty for gaming, really.
But if you've made your mind up on having a few extra cores from AMD,
I'll suggest you go with this AMD Athlon II X4 640
& save yourself some money or invest it in a high end GPU like the Radeon HD 5870
Also you need'nt even have bought a 650 watt PSU if you are not planning to Xfire any time soon