$500 should be plenty to buy a bare-bones pre-built off the shelf. You can always upgrade individual pieces later. As long as the system board and CPU are decent, you have a load of upgrade potential.
Other pieces that are easily transferred between systems:
Sound / NIC / SATA cards
GPU's
Power Supplies
Physical Drives (HD / DVD / Disc)
Other pieces that are easily transferred between systems:
Sound / NIC / SATA cards
GPU's
Power Supplies
Physical Drives (HD / DVD / Disc)