A HD7870 ghz edition would be very optimal for you rig. It has some of the best performance/watt ratio out there according to this source:
http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_Titan/28.html
It beats the titan & 690 in most cases in perf/watt, if you go on to the next page, it also has more value too!
An Nvidia counterpart would also be smart since you are using a Nvidia card atm so you wouldn't have to go through the process of uninstall driver and install new drivers, etc... A GTX 650 ti boost or GTX 660 both are good even though the HD7870 ghz outperform most of the time according to this source:
http://gpuboss.com/gpus/Radeon-HD-7870-vs-GeForce-GTX-660
Another note that you should take in. It is a shame that most of games that we play these day are console ports from the xbox360, in the near future of all 3 next-gen console will house AMD APU (GPU in Wii U) so having a GCN GPU would "technically" help in some ways i believe. This is pure opinion.
If you want to bitmining, this GPU can technically mine and pay for itself.
Hope i help you
p/s: i just help this guy on choosing the HD 7870 ghz. You can check out my contribution to it! link:http://www.tomshardware.com/answers/id-1668178/gtx-650-radeon-7850.html