I don't think 500 bucks is enough for OS , Monitor , Keyboard , mouse and the system itself However I did one for 525 without a monitor
PCPartPicker part list:
http://pcpartpicker.com/p/yQrXVn
Price breakdown by merchant: http://pcpartpicker.com/p/yQrXVn/by_merchant/
CPU: Intel Pentium G3258 3.2GHz Dual-Core Processor ($69.29 @ Amazon)
Motherboard: Gigabyte GA-B85M-DS3H Micro ATX LGA1150 Motherboard ($56.99 @ Newegg)
Memory: G.Skill Ripjaws Series 4GB (2 x 2GB) DDR3-1600 Memory ($45.00 @ Newegg)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($55.98 @ OutletPC)
Video Card: Gigabyte Radeon R7 260X 2GB Video Card ($109.99 @ Newegg)
Case: Enermax ECA3253-BW ATX Mid Tower Case ($34.99 @ Amazon)
Power Supply: EVGA 500W 80+ Bronze Certified ATX Power Supply ($34.99 @ NCIX US)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($89.98 @ OutletPC)
Keyboard: Cooler Master CM Storm Devastator Gaming Bundle Wired Gaming Keyboard w/Optical Mouse ($27.81 @ NCIX US)
Total: $525.02
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2014-08-20 22:46 EDT-0400