Newegg.com - Intel Core i5-3570K Ivy Bridge 3.4GHz (3.8GHz Turbo) LGA 1155 77W Quad-Core Desktop Processor Intel HD Graphics 4000 BX80637I53570K
Newegg.com - ASRock B75M-DGS LGA 1155 Intel B75 SATA 6Gb/s USB 3.0 Micro ATX Intel Motherboard
Newegg.com - Rosewill CHALLENGER Black Gaming ATX Mid Tower Computer Case, comes with Three Fans-1x Front Blue LED 120mm Fan, 1x Top 140mm Fan, 1x Rear 120mm Fan, option Fans-2x Side 120mm Fan
Newegg.com - G.SKILL Ripjaws Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL9D-8GBRL
Newegg.com - Computer Hardware, Video Cards & Video Devices, Desktop Graphics Cards, PCI Express 3.0 x16, $100 - $200
This would run you around $600. If you have the HDD, PSU and OS you would be set. Just pick the card you prefer, ATI/AMD or NVIDIA. Personally, I have the 2GB version of the 7850 and it is killer, but also $200. I run a quad core Phenom 2 which is $100 cheaper, but not nearly as good a processor as the 3570k. If I had to build a new system all over again this is what I would go for.