This is basically what I went through to get the Gigabyte R9 280X working in my upgraded 2009 Mac Pro and enables me to get the boot screen. You can install this card with the correct power cables which it does not come with and it will work but to get the boot screen the card must be flashed. Here is a great guide about flashing pc cards to make them mac compatible. FYI the R9 280x is really just a re branded AMD 7970 with a few tweaks.
Also www.nekas.org is great for info on flashing cards for mac, become a member and read in the forums a lot before posting questions, this site is where the video card gurus post info about flashing.
Check this out. The GTX 980 outperforms the R9 280X they have been playing around with R9 290X as well.
Took me a while to get this video up so as time passes the cards keep a comin.
Again I am not an expert on this and had a lot of help to get my card working so just putting this video out there for folks who may want to take the leap and upgrade their aging video cards, also check out macvidcards store on ebay, they sell flashed cards for older mac pros and will flash a card for you for a fee. I wished I had just ponied up the bucks initially because I kind of went through a lot and two different cards to get his baby working.