But to quickly answer your question regarding CPU:
"The good news is that any modern dual-core CPU over 2.0 GHz should be able to run the game acceptably, and more megahertz will only help."
As far as your system build, you should check out the "build your own" forum, here on TH. they have a lot of great stickied tutorials and whatnot.
My advice (and i am just a newbie, so dont take my word for it), would be to try and save a little bit more money. $500 isnt much, though I am sure you can build a solid gaming rig for that much. I just think an extra $100-200 dollars would get you a much better system for not much more money. but i understand what its like being on a strict budget.