Well, a 7-year pc should definetely be upgraded with something special.
I'd say..... go for an r9 280X or an R9 290-290X and built a pc around that.
There are already great price offers online for new and "like new" cards like the ones i mentioned, and they are beasts of a card with plenty of performance.
The other solution is to post on the nvidia forum and get great alternatives there as well...
gigabyte X58A-UD3R rev.2, core i7 975 3.9ghz (stock voltage), windows 8.1 64-bit, corsair 12gb DDR3 1600mhz 7-7-7-20 RAM, xfx HD 5970 + gigabyte HD 5970 (950mhz/ 1100mhz), coolermaster ultimate 900w PSU.
amd catalyst 14.12 Omega Driver