Give a full list of specs + budget. From what you provided so far, I would recommend:
- R9 280 / R9 280X / GTX 770 (the upgrade from a GTX 560 to a 270X isn't worth the money).
- Stay with 8GB RAM. If you use more than 6GB RAM for more than half the day, then go for 12GB (just add a single 4GB stick). If you max out 8GB RAM for more than half the day, then go for the 16GB RAM. If you buy more ram, make sure the voltages and CAS latency match up with your current RAM.
- 750W is overkill. You do not need 750W for a single GPU. Most you need is probably 500W; 600W to be safe.
- If you don't already, I would consider getting an SSD.
TL

R - Add SSD, get R9 280/280X or GTX 770.