Look up bare bones kits on Newegg. They've got all the parts you need to get you started, you just have to put them together. Or if you do a little research to find out what parts work together, you can build your own. I have a friend with a $~300 dollar budget, and he's going to get a cheap AMD quad core AM3 socket processor (A decent one) (80-100), a cheap board (70-100), decent ram (50), and a decent video card (100-150).
So, if you have 300, you can make it, but barely. 400 it's a lot easier, and 500 is an ideal range.