For your system, I think you should invest in the future and get something like a GTX 1050Ti. It will give you a boost in performance for the time being, and you'll be able to carry that GPU forward in a new build.
If you purchased something like a GTX 5 or 6 hundred series, you'll be facing another new GPU in a couple of years if you decide to build a new system from scratch.
That's a very old board. Depending on your CPU, you may be looking at something older like a GTX 500 series or 600 series. However, the GTX 1050 may work just fine.
But I think an overhaul is probably where your investment should go. There is no future with this board.
It has a PCIe x16 slot. It should technically support any PCIe card on the market. Including a GeForce GTX 1080 Ti.
Given that at best you have a Core 2 Quad or Xeon with a 771 to 775 pin adapter installed. I wouldn't install anything beyond a GeForce GTX 1050 Ti. Prefereably a Radeon Rx 460 2GB due to the limited CPU speed. You will get diminishing returns on a more expensive GPU. Especially with only 2GB RAM.
Frankly, given how old the system is and the limited amount of RAM. If you have a good power supply. I'd look at going used. A used GeForce GTX 660 2GB should handle any game that old CPU and small amount of RAM could run. Heck, a GTX 560 Ti 1GB is probably plenty.
For your system, I think you should invest in the future and get something like a GTX 1050Ti. It will give you a boost in performance for the time being, and you'll be able to carry that GPU forward in a new build.
If you purchased something like a GTX 5 or 6 hundred series, you'll be facing another new GPU in a couple of years if you decide to build a new system from scratch.