It depends a lot on what you are going to use it for. You have to consider the speed of the chip, the amount of RAM on the card, if you're going to use it for video games or graphics design on 4k monitors, you have to consider the buffer memory. If you plan to do GPU programming, especially on nvidia, the version of CUDA you use is important as it determines the number of threads that can be executed in a kernel.

But if you're not doing anything special, and you're looking for something that won't be obsolete in a short time, any GTX 900 or better card will do (or AMD equivalent).

The main thing would be which video card port the computer motherboard has (PCI, PCI-Express or AGP), after determining that decide which brand you want to buy based on the intended use and the Operating System in which the graphics card will be used.