Well , if you really wanna upgrade, an upgrade would be either Gtx 780 or a r9 290x.
r9 290x runs really loud though, so you might wanna consider that.
If you want to shelve out 700 dollars for a card, then perhaps there is none better than Gtx 780ti
Yes, if you crossfire, 7950 and r9 290 , you wouldn't get any obvious benefit.
And you can't run two cards ATI and NVIDIA on the same machine