Currently planning on building a Pc for gaming

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: AMD FX-6300 3.5GHz 6-Core Processor ($113.48 @ OutletPC)
Motherboard: ASRock 970 Extreme3 R2.0 ATX AM3+ Motherboard ($59.99 @ Microcenter)
Memory: G.Skill Ares Series 8GB (2 x 4GB) DDR3-1600 Memory ($69.99 @ Newegg)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive ($59.98 @ OutletPC)
Video Card: MSI Radeon R7 260X 2GB Video Card ($109.99 @ Newegg)
Case: BitFenix Shinobi Window ATX Mid Tower Case ($49.99 @ NCIX US)
Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply ($49.99 @ NCIX US)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($89.98 @ OutletPC)
Total: $603.39
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-03-03 01:22 EST-0500)

-Stronger processor.
-1tb HDD for storage.
-Better quality case and psu.
-Stronger gpu.
 
Solution

Bad_Kitty13

Admirable
PCPartPicker part list: http://pcpartpicker.com/p/33eAD
Price breakdown by merchant: http://pcpartpicker.com/p/33eAD/by_merchant/
Benchmarks: http://pcpartpicker.com/p/33eAD/benchmarks/

CPU: Intel Core i3-4330 3.5GHz Dual-Core Processor ($133.97 @ OutletPC)
Motherboard: Asus H87M-E Micro ATX LGA1150 Motherboard ($93.24 @ Amazon)
Memory: G.Skill Ares Series 8GB (2 x 4GB) DDR3-1600 Memory ($77.98 @ OutletPC)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($59.99 @ Amazon)
Video Card: Gigabyte GeForce GTX 750 1GB Video Card ($119.99 @ Amazon)
Case: NZXT Source 210 (Black) ATX Mid Tower Case ($29.99 @ Microcenter)
Power Supply: SeaSonic 300W 80+ Bronze Certified ATX Power Supply ($39.99 @ Mwave)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($92.99 @ B&H)
Total: $648.14
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-03-03 01:30 EST-0500)

i3 is good processor for gaming
750 is a big graphics increase
more reliable psu
very good mother board
better storage solution for your price range

here are some bench marks of the 750
http://www.anandtech.com/show/7764/the-nvidia-geforce-gtx-750-ti-and-gtx-750-review-maxwell/13