You can get this
http://www.macvidcards.com/store/p45/Nvidia_GTX_970_4_GB.html
Or this
http://www.macvidcards.com/store/p24/AMD_Radeon_R9_280X_3_GB.html
under $600 and has some sort of 4k / 5k support.
For these cards (I mean from MVC), you don't have to flash it by yourself. He did all the mods for you, and the card can run with the Mac Pro's internal power only safely.
Both of them stronger than 5770x2, and good for gaming.
For Nvidia card. It required OSX 10.10 or above. Require you to install AND activate the driver. It's a better card, faster and more energy efficient when compare to the 280X. However, it's not that fun to deal with the driver especially after an OS upgrade. Nvidia usually release new driver in few days, however, if you accidentally upgrade the OS before they release the driver, it may cause you some trouble.
For the 280X, it works just like an official Apple GPU. With boot screen, no need to manage drivers (it also means AMD won't give you any better driver than the Apple stock driver), just plug and play. However, it's old tech when compared to the 970, and it consume more power with less gaming performance.
TBO, I never use any Nvidia card, so I don't know how it works in FCPX. But AFAIK, in general, FCPX works better with AMD card, and 970 better for gaming.
Also, there is no GPU (including the D700 in new Mac Pro) has prefect support for 4k / 5k yet (especially above 4K 60Hz). They may or may not give you boot screen (depends on monitor and connection type). They may or may not boot with the monitor connected (power on). And possible artifact on some models. So, for me, this will be the very last thing to concern when I choose my new GPU. Because there is no guarantee anyway. And better to do some study before you buy the monitor, some models work much better than the others.
On the other hand. There are few more choices if you don't need the boot screen (This boot screen doing nothing when everything is normal, however, it's extremely useful when something goes wrong). e.g. a PC HD7950 or PC GTX 680. As long as you get the reference card (2x6pin), they should work OOTB, no driver required (even though you can still install Nvidia driver for the 680, but it's not required, unless you need CUDA). These are the much cheaper option, and you can simply keep one of the stock 5770 for emergency / backup use, which will give you boot screen, and as a backup GPU.
I just get a new GPU a month ago.
https://forums.macrumors.com/thread...tem-information.1808938/page-14#post-21607156
It works very well so far. I flashed it by myself to get the boot screen. It's quiet, easy and safe to flash (even though you won't do it now, you may do this in future), within the Mac Pro power limit, etc. And it just cost me $150 (new card). This card is same as the 7950, just a new name from AMD. So it will also work OOTB without flash.
If you want a full feature Mac compatible card, and don't want to flash it by yourself. I suggest you get a card from MVC, he is a trustable source, and he will provide support after your purchase. Which cost you more, but worth. On the other hand, if you just looking for performance to cost ratio, then a PC card is the way to go. But I personally suggest you do more study before making any decision if you go for this route.