Looking at flashed ones on eBay - what would you recommend will work most reliably on an el cap 5,1? Ive found a 3Gb 7950 that looks decent - any others better? Thanks for your help!
No need to go for a flashed card
1) It's more expensive
2) You can flash it by yourself if you want to
3) The MOST important one, it won't gives you 4K 60Hz boot screen, but just hang you computer.
Then why pay more to buy trouble?
There is NO 4K 60Hz OOTB card with boot screen exist on the world yet (for cMP).
For El Capitan, you only have one choice for 4K 60Hz boot screen. A flashed Maxwell card from MVC. However, it require Nvidia web driver to work, definitely not a OOTB solution.
And all other GPU either not working in El Capitan at all. Or cannot provide 4K 60Hz boot screen.
Therefore, only go for normal PC card, that's the best you can do. HD7950 is a good choice for just driving 4K 60Hz OS UI.
There is not much better choice, 7970 is stronger, but power management may be a bit messy.
GTX680 is also a good choice, but depends on usage, it's not necessary better than 7950 (especially if you use it for compute).