Triple Core CPU: AMD Phenom II X3 715 Black Edition Heka for $76 You may be able to unlock this to get a quad core out of it. Worth a try, and even if you are not successful, you will still not be CPU constrained for some time to come.
Total is $296 after rebate, promo codes and shipping. This is just about the rock bottom build that I would recommend. You could go cheaper on the CPU/Cooler, if you need to get Windows with it, but that would take you over $300. Anything less, and you will start to run into junk parts.