What is the best 800$ build ? For gaming

Solution
PCPartPicker part list: http://pcpartpicker.com/p/rZ7BhM
Price breakdown by merchant: http://pcpartpicker.com/p/rZ7BhM/by_merchant/

CPU: Intel Core i5-4670 3.4GHz Quad-Core Processor ($209.98 @ SuperBiiz)
Motherboard: Asus H97-PLUS ATX LGA1150 Motherboard ($105.24 @ Amazon)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-2400 Memory ($82.98 @ OutletPC)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($54.98 @ OutletPC)
Video Card: MSI Radeon R9 270X 2GB TWIN FROZR Video Card ($179.99 @ Amazon)
Case: NZXT Source 210 (Black) ATX Mid Tower Case ($37.99 @ Micro Center)
Power Supply: XFX ProSeries 450W 80+ Bronze Certified ATX Power Supply ($61.98 @ SuperBiiz)
Optical Drive: Samsung SH-224DB/BEBE...
I just ordered this last week from the egg. You could tweak it around to your preference if you want stronger in one area and less in another. I was keeping it low wattage :D

PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i3-4360 3.7GHz Dual-Core Processor ($154.99 @ NCIX US)
Motherboard: ASRock H97M PRO4 Micro ATX LGA1150 Motherboard ($84.98 @ Newegg)
Memory: G.Skill Ripjaws Z Series 8GB (2 x 4GB) DDR3-2400 Memory ($84.99 @ Newegg)
Storage: PNY Optima 240GB 2.5" Solid State Drive ($89.99 @ Newegg)
Video Card: MSI GeForce GTX 750 Ti 2GB Video Card ($119.99 @ Newegg)
Case: Xion XON-560 MicroATX Mini Tower Case ($46.98 @ Newegg)
Power Supply: SeaSonic 300W 80+ Bronze Certified ATX Power Supply ($42.99 @ Mwave)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($14.98 @ OutletPC)
Operating System: Microsoft Windows 8.1 (OEM) (64-bit) ($89.98 @ OutletPC)
Keyboard: Microsoft 7ZJ-00001 Wired Ergonomic Keyboard w/Optical Mouse ($21.97 @ Mwave)
Total: $751.84
Prices include shipping, taxes, and discounts when available

Your list is ok but get a quality PSU.
 
PCPartPicker part list: http://pcpartpicker.com/p/rZ7BhM
Price breakdown by merchant: http://pcpartpicker.com/p/rZ7BhM/by_merchant/

CPU: Intel Core i5-4670 3.4GHz Quad-Core Processor ($209.98 @ SuperBiiz)
Motherboard: Asus H97-PLUS ATX LGA1150 Motherboard ($105.24 @ Amazon)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-2400 Memory ($82.98 @ OutletPC)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($54.98 @ OutletPC)
Video Card: MSI Radeon R9 270X 2GB TWIN FROZR Video Card ($179.99 @ Amazon)
Case: NZXT Source 210 (Black) ATX Mid Tower Case ($37.99 @ Micro Center)
Power Supply: XFX ProSeries 450W 80+ Bronze Certified ATX Power Supply ($61.98 @ SuperBiiz)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($14.98 @ OutletPC)
Total: $748.12
Prices include shipping, taxes, and discounts when available
 
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