What should i pick (pc build)

[Messinian]

Hello I am trying to build a pc, I am getting some of the parts from the person who is helping me.. I am a noob to pc building. I may be a noob to building but i still know hardware ok. I was given a asus gaming mother board with am3+ socket. I was also given 8gb of Rip jaws x ram and a 550watt power supply. Not sure of the brand and I was given a case. and a corsair liquid cooler for the cpu. I need to get a cpu and gpu. What are my best options, I have the money to buy whatever but im cheap and need to buy a fourwheeler.(only reason i have a bunch of money is i sold my dirtbike to get a wheeler). Id like to spend around 300 for the pair. Will the amd fx 6300 be good? Ive heard that it can be overclocked like crazy and i have a liquid cooler for it already. What about gpus? is the msi r9 270x any good for gaming? or will the 550watt power supply be enough for a r9 280x? I would like to play games like battlefield 4 and black ops 1(so fun to play because of all the mods available for it.)(niot hack mods but like minecraft mods in the way they change the game to something else.) Thanks

 

[Mac266]

PCPartPicker part list / Price breakdown by merchant

CPU: AMD FX-8320 3.5GHz 8-Core Processor ($143.98 @ OutletPC)
Video Card: Sapphire Radeon R9 290 4GB Vapor-X Video Card ($491.13 @ Newegg)
Power Supply: XFX ProSeries 750W 80+ Gold Certified Fully-Modular ATX Power Supply ($107.99 @ NCIX US)
Total: $743.10
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2014-06-17 01:08 EDT-0400

Included a recommended PSU, a no-name unreliable PSU will kill your system.