Hmmm, $500 will buy you about half of a good computer, (before everyone starts posting how you can get 'good' computers for $600 let me clarify...) by 'good' I mean an actual high-end pc.
Here's a few prices for you to think about
Graphics Card: ~$200
CPU: ~$180
MOBO: ~$130
PSU: ~$110
HDD(s): ~$90
Optical Drive: ~$30
RAM: ~$120
Monitor: ~$150
Case: ~$80
What are you planning to use your new pc for? Gaming? Work? Browsing the interwebs? Post up what you need and someone will put together the cheapest build they can for what you want