First of all, use an online site like Newegg.com, NCIX.com, or other in your country / region; they'll stomp all over Worst Buy prices. If there is a Microcenter in your area, they have great in-store deals, typically including CPU/Mobo combos.
Take a look at this quarter's $600 SBM build to get an idea of what $600 can buy (the remaining $100 of your budget can go for Windows). This assumes, of course, that you already have a monitor, mouse, keyboard, and speakers. If you don't have those, you'll need to make some significant adjustments.
The best thing you can do is fill out the template in the sticky on How to Ask For New Build Advice. It will answer these (and many other) questions, and help you get the most relevant answers.