Gaming PCs varies.
For $750 and 1080p gaming in the U.S., I would recommend:
CPU: Either AMD Fx 8320 ~$140 OR Intel i5 4690k ~$240 (The i5 has faster single core performance but very little advantage in FPS for extra $100, I would recommend going with FX 8320, overclock it, and spending the extra $100 on gpu, which it'll make a bigger impact.)
Mobo: Gigibyte 970A-ud3p (the best 970 chipset mobo with 8 power phases, good for easy OC) ~$70 / Intel: look for a good 1150 z97 board ex: MSI Z97 GAMING 5 LGA 1150 Intel Z97 ~$125
GPU: (For AMD cpu) R9 290x or GTX 970 whichever has better deal, around $320 / (For Intel cpu) R9 380 or GTX 960 ~$190.
Ram: You already have.
HDD: 1T 7,200 HDD ~$45 / optional add SSD if you go with AMD and cut down on gpu.
PSU: (very important! Don't cheap out) EVGA 750w B2 ~$80
Case: Varies $30 and up.
AMD = ~U.S. $655 + case + cpu cooler Or Intel = ~$680 + case + cpu cooler.
I personally would pick AMD over Intel, despite its obvious disadvantage, you are better off with the extra money for the GPU. For a gaming PC, the most powerful component should be your video card.