ExtremeTech published a guide on building a $350 computer system that runs Windows Vista, that's without a display of course. Unfortunately they didn't include some benchmarks.
Now, if you're looking to become the next Martin Scorsese or beat the high score in Crysis, then read no further. But maybe your requirements are a lot simpler. Maybe you're the tech-savvy type who owns a power rig but would like a second machine for general use in the den. You could have a child or children looking for their first computer. You may be a college student who needs a computer with just the basics; or you could be starting your own home business and want to keep computing costs to a minimum. In any of these scenarios, a budget PC will be more than sufficient to meet your needs. The fact is, you can easily put together a very inexpensive PC with little to no scrounging around for parts. All the components we used in our Build It project are easily accessible on sites like Newegg.com, and are surprisingly easy to assemble.