Let me try to give you an answer that explains what you can use with Yosemite against what you can use with Sierra and I will try to point you towards some options that will suit different games.
Yosemite is now a few years old and the hardware has moved on. Apple compatible GPU models are unlikely to be available as a new option to purchase off the shelf.
The nvidia cards are the better option to consider. The cards that will work for 10.10.5 require additional nvidia drivers (see Downloads section) for further details. The GTX 950, GTX 960, GTX 970, GTX 980 and GTX 980Ti (Maxwell) cards will all function in Yosemite. The level of card that you choose will depend on your gaming requirements, budget and the local availability/pricing of used cards.
A GTX 950 would suit some basic gaming, either using Yosemite or dual booting into Windows. If you are a more serious gamer or you are looking at some more modern gaming titles, then you may find that the GTX 970/GTX 980 options are more appropriate for 1080P gaming. If you use Steam, then check your games recommended GPU cards and don't go below these recommendations as newer titles will require better (higher) specifications to take advantage of.
https://www.tonymacx86.com/buyersguide/march/2016#Graphics_Cards
If you are willing to move to Sierra to get the newest card models then the GTX, 1050, GTX 1060(3GB & 6GB), GTX 1070 and GTX 1080 (Pascal) cards can be considered (see latest buying guide).
You should be able to run a GTX 980 or GTX 1080 with your existing PSU - if you find that the PSU fan is noisy and distracting, then at that point it would be worth moving to a replacement PSU.
Performance difference between different graphics cards:
https://www.tonymacx86.com/threads/...-2017-hierarchy-chart-by-toms-hardware.81325/
The general 1080P gaming sweet spot is probably around the GTX 980/980Ti Maxwell cards and GTX 1060/GTX 1070 Pascal cards for the bigger title games. You may find that if you play mainly indy titles then you can go a little lower.
