If a video card is not specifically labelled as Mac compatible then this almost certainly means it does not have Mac compatible firmware as standard. This does not mean it will not work, it means that it will not show anything until the Mac has finished booting and loaded the drivers needed to use it. Until it has finished booting and loaded the drivers you just get a blank screen. This also means you have to have the right drivers installed on the Mac before fitting the card.
For Nvidia cards like the GTX 980 you need to yourself download and install the drivers before fitting the Nvidia card. The latest Nvidia drivers which are needed for the GTX 980 and 970 only work with Yosemite.
For AMD cards like the Radeon 7950, the Radeon 7970, the R9 280, and the R9 390 the drivers are only available from Apple and will only be included in a version of OS X that post-dates the video card. So the Radeon 7950 and 7970 are now quite old cards and Apple added drivers for them a long time ago - I believe this was with OS X 10.8.3. The R9 390 will likely require a much newer OS X version.
It is possible to 'flash' both Nvidia and AMD cards to add Mac firmware. With Mac firmware you will see something on screen as soon as you turn the computer on, and you will then also see the boot process.
For the Nvidia GTX 980 and 970, you need to get a company called MacVidCards to do this for you - see http://www.macvidcards.com/store/c2/Nvidia_GPUs.html
For AMD video cards like the Radeon 7950, 7970, the R9 280 and the R9 390 you can in theory do-it-yourself. This involves using utilities in Windows and could be done in a 'real' PC but most often is done using Boot Camp. It maybe that you need to have a second older properly Mac compatible video card fitted at the same time to use while flashing the new card.
So as has been said many, many times in these forums, if you just buy an off the shelf video card it will initially not show anything on a Mac until you have the drivers loaded which presumes you have the drivers already installed.