As a general build sans case, psu, and hdd, this is what I would put together for $500.
PCPartPicker part list: http://pcpartpicker.com/p/pfcR
Price breakdown by merchant: http://pcpartpicker.com/p/pfcR/by_merchant/
CPU: Intel Core i5-3570K 3.4GHz Quad-Core Processor ($199.99 @ Newegg)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($24.99 @ Newegg)
Motherboard: ASRock Z77 Extreme4 ATX LGA1155 Motherboard ($104.99 @ Newegg)
Memory: Samsung 8GB (2 x 4GB) DDR3-1600 Memory ($34.99 @ Newegg)
Video Card: EVGA GeForce GTX 650 2GB Video Card ($139.99 @ Newegg)
Optical Drive: Lite-On iHAS124-04 DVD/CD Writer ($15.99 @ Newegg)
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2012-11-24 13:54 EST-0500)
This will give you some serious processing power, a GPU that can still hold it's own weight easily, and easy OC potential with a K series CPU, a great cooler, and amazing RAM.
Drawbacks of this set is that it leaves you no money room for an OS, your case will need to come with fans, all of your cables need to be included with your components, and you'd need your PSU to be at least enough to run the equipment.