it seems as though your 6670 (r7 250) is the bottleneck. even though i hold that unit in esteem as a budget + composite use resource spartan card that compliments the AMD APU well, if it comes down to gaming with a fx cpu, it is holding back your pc from its full potential.
the 7850 (r9 270?) is what is most popular that users prefer for the price/performance end result . last i checked, the low end OEM (MSI, XFX, PNY, HIS) 7850s are in the 200 USD range. if a few more bucks can be parted with, the 7870 or 7950 is within reach. i prefer Visiontek and Asus OEM GPUs myself, but other seasoned users may have their preferences also.
also note that your power supply is in the equation as well. the 6670 is the most efficient card concerning the PSU and the wattage/amperage curve climbs from there.