I've attached a PDF file for a system i put together for you at newegg.com
You are buying parts and will have to put it together yourself but its actually very straight forward and simple. The instructions included with the parts are easy to follow.
The system will work out to be $329.92 after all the mail in rebates and stuff. Good system that will be able to do most everything. If you drop another 100 bucks for a good video card, it will be a really good budget gaming system.
You will still need to purchase an operating system, i would recommend Windows 7 Home Premium 32-bit. Make sure you DO NOT get the retail copy, get an OEM copy and you can save yourself a hundred bucks.