CPU - There is no reason to go with the G3420 over the G3220. The 200MHz difference will not be noticed. If you want to play around with overclocking, look toward the G3258. The better option would be the i3-4160.
CPU COOLER - Only needed if you want to overclock the G3258. Drop this if you go with the G3220 or other non-overclocking CPU as the stock Intel cooler would be fine.
MB - B85 is OK, but the H97 chipset is the way to go with its newer feature sets.
MEM - Try to go with 8GB up front. 4GB is a bare minimum...
GPU - The HD 7870 is a fine GPU. A little dated, but still very capable.
CASE - Changed to a solid budget oriented case.
PSU - No. XFX, Seasonic, EVGA G2 or filter through the Tier 1 or Tier 2 list. The Corsair C and Builder series are fairly poor units (cheap capacitors). www.tomshardware.com/forum/id-2547993/psu-tier-list.html
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel Pentium G3258 3.2GHz Dual-Core Processor ($64.88 @ OutletPC)
CPU Cooler: Cooler Master Hyper TX3 54.8 CFM Sleeve Bearing CPU Cooler ($19.89 @ OutletPC)
Motherboard: ASRock H97 PRO4 ATX LGA1150 Motherboard ($75.98 @ Newegg)
Memory: Crucial Ballistix Sport 8GB (2 x 4GB) DDR3-1600 Memory ($37.99 @ Amazon)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($50.89 @ OutletPC)
Video Card: MSI Radeon HD 7870 GHz Edition 2GB Video Card (Purchased For $0.00)
Case: Deepcool TESSERACT SW ATX Mid Tower Case ($43.98 @ Newegg)
Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply ($55.99 @ SuperBiiz)
Optical Drive: Asus DRW-24F1ST DVD/CD Writer ($12.99 @ Newegg)
Total: $362.59