how can i upgrade a gateway dx 4380g for gaming

Solution
Here's my recommendation :

PCPartPicker part list: http://pcpartpicker.com/p/1CDKk
Price breakdown by merchant: http://pcpartpicker.com/p/1CDKk/by_merchant/
Benchmarks: http://pcpartpicker.com/p/1CDKk/benchmarks/

CPU: Intel Core i5-4570 3.2GHz Quad-Core Processor ($189.99 @ SuperBiiz)
Motherboard: MSI B85-G41 PC Mate ATX LGA1150 Motherboard ($78.23 @ Outlet PC)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory ($59.98 @ NCIX US)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($59.99 @ NCIX US)
Video Card: Gigabyte Radeon HD 7950 3GB Video Card ($203.98 @ Newegg)
Case: Corsair 200R ATX Mid Tower Case ($39.99 @ Newegg)
Power Supply: XFX 550W 80 PLUS Bronze Certified ATX12V / EPS12V...
Here's my recommendation :

PCPartPicker part list: http://pcpartpicker.com/p/1CDKk
Price breakdown by merchant: http://pcpartpicker.com/p/1CDKk/by_merchant/
Benchmarks: http://pcpartpicker.com/p/1CDKk/benchmarks/

CPU: Intel Core i5-4570 3.2GHz Quad-Core Processor ($189.99 @ SuperBiiz)
Motherboard: MSI B85-G41 PC Mate ATX LGA1150 Motherboard ($78.23 @ Outlet PC)
Memory: G.Skill Ripjaws X Series 8GB (2 x 4GB) DDR3-1600 Memory ($59.98 @ NCIX US)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($59.99 @ NCIX US)
Video Card: Gigabyte Radeon HD 7950 3GB Video Card ($203.98 @ Newegg)
Case: Corsair 200R ATX Mid Tower Case ($39.99 @ Newegg)
Power Supply: XFX 550W 80 PLUS Bronze Certified ATX12V / EPS12V Power Supply ($44.99 @ NCIX US)
Optical Drive: LG GH24NS95 DVD/CD Writer ($15.98 @ Outlet PC)
Operating System: Microsoft Windows 8 (OEM) (64-bit) ($88.88 @ Outlet PC)
Total: $782.01
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-09-11 17:38 EDT-0400)
 
Solution