I beg to differ.
It is a bit difficult to buy a good gaming pc for that price, but it is limited.
You just need to know where to buy the parts (and make sure it's all compatible.)
OS: Windows 7/8 (Your choice) $100
Processor: Try an Intel core i3. It's not the fastest but it'll get the job done. $100
RAM: 4gb. I would recommend Crucial Ballistix Sport $40
Hard drive: 1TB (Western Digital or Barracuda) $80
Motherboard: Try and find something that's about $60 (preferably gigabyte)
Case: your choice. Try and find something that comes with a power supply. You can knock out 2 birds with 1 stone on that. $50
Total Price: About $430 (plus tax)
Have fun!