This isn't usually the best place for this question, but you might take a look at the system builder articles. The have a budget build that costs $650 (probably a little less today). You could probably upgrade the CPU from the listed i3 to an i5, and maybe even get a little better GPU as well.
Here is their budget build: http://www.tomshardware.com/reviews/build-gaming-pc-overclock,3159.html
However, you are likely best off looking here for input: http://www.tomshardware.com/forum/forum-31-202.html
It's a forum section dedicated to helping build systems.