Yep, it's an old rig alright.
Personally I'd hold off on getting a big card now, with Pascal and Polaris (Nvidia and AMD) due fairly soon you're going to end up putting an 'obsolete' card into the new build.
If the 8800 is dead/passing away you could drop a cheap replacement like the R7 250 or GT740 in and wait for the Pascal/Polaris cards from Nvidia/AMD to turn up, either should run on a non UEFI BIOS system and neither draws as much power as the 8800.
OR you could hunt through E-bay/Craiglist for something used, just be sure that the power supply can handle any card you have in mind, older cards can have a serious thirst for power.