Here are a few boutique builder options.
Fairly close to the build listed above, less GPU horsepower:
http://www.newegg.com/Product/Product.aspx?Item=N82E16883229437
APU computer (CPU/GPU on one chip, you can add a video card later if you need more oomph)
http://www.newegg.com/Product/Product.aspx?Item=N82E16883227508
Don't like listing this one since it is a bit of a compromise. Adequate Quad core CPU with the better GPU out of the pre-builts listed above.
http://www.bestbuy.com/site/desktop-8gb-memory-500gb-hard-drive/6979787.p?id=1218818468711&skuId=6979787&st=pcmcat287600050002_categoryid$abcat0501000&cp=1&lp=15
Here is a decent build, OS included, and a fairly powerful GPU. From only Amazon and Newegg. It is more expensive to go with just one of them.
PCPartPicker part list:
http://pcpartpicker.com/p/1YqcF
Price breakdown by merchant: http://pcpartpicker.com/p/1YqcF/by_merchant/
Benchmarks: http://pcpartpicker.com/p/1YqcF/benchmarks/
CPU: AMD Athlon II X4 760K 3.8GHz Quad-Core Processor ($77.00 @ Amazon)
Motherboard: ASRock FM2A55M-DGS Micro ATX FM2 Motherboard ($49.99 @ Newegg)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory ($63.75 @ Newegg)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($64.59 @ Amazon)
Video Card: EVGA GeForce GTX 650 Ti Boost 1GB Video Card ($129.99 @ Amazon)
Case: NZXT Source 210 (Black) ATX Mid Tower Case ($34.72 @ Amazon)
Power Supply: Corsair CX 430W 80 PLUS Bronze Certified ATX12V Power Supply ($24.99 @ Newegg)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($67.99 @ Amazon)
Total: $513.02
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-11-06 17:36 EST-0500)