I doubt you'll be able to assemble a decent gaming system for a max of $500. You're looking at minimum
- $200 for the processor,
- $100 for the motherboard,
- $100-$150 for a video card,
- $50-$80 for ram (I'd recommend at least six to eight gigs).
- Hard drive, $80
- Power supply $60-$80
- Case $50+
- Screens, $100+ each.
- Speakers (using default sound card on mobo), $20+
- Mouse and keyboard, $30
Do you have parts from your current computer you can use? If your hard drive is over 5200rpm it may be a good idea to swap that. If you have a decent screen that can save you some money.