Drop down to the 8320 and get a smaller PSU, should let you scrape enough for a R9-270X or even a GTX760.
You really only need about 550-600W for that machine with a single GPU.
Didn't quite realize how much that HX750 was going for, unless you are getting a deal, there are cheaper supplies out there with similar performance.
PCPartPicker part list:
http://pcpartpicker.com/p/Hz3RnQ
Price breakdown by merchant: http://pcpartpicker.com/p/Hz3RnQ/by_merchant/
CPU: AMD FX-8320 3.5GHz 8-Core Processor ($139.99 @ Amazon)
Motherboard: ASRock 970 Extreme3 R2.0 ATX AM3+ Motherboard ($59.99 @ Newegg)
Memory: Corsair XMS3 8GB (2 x 4GB) DDR3-1600 Memory ($72.99 @ Newegg)
Video Card: Asus GeForce GTX 760 2GB DirectCU II Video Card ($223.99 @ Newegg)
Power Supply: EVGA SuperNOVA NEX 650W 80+ Gold Certified Fully-Modular ATX Power Supply ($59.99 @ Newegg)
Total: $556.95
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2014-09-10 17:28 EDT-0400