It seems to me like the best value for little endian is an RX 470 or 570, a used one on eBay goes for rougly $200 to about $250. More than enough power for desktop and video stuff, and still plenty powerful for 3D graphics and the few video games that are currently available for POWER. For just desktop stuff, again without needing BE support, an HD 8490 is like $20 and seems like it should suffice for desktop acceleration and video streaming.
If you're a big-endian die-hard, an HD 6970 is pretty much the cream of the crop according to my digging. An HD 6990 might work but it's only generally competitive with the 6970 (and on par with a 6950 without Crossfire) for a hugely higher power draw. Of course, previous gen cards from that should work too, but I'd definitely say that with how inexpensive the 6970 is (I see them going on the 'Bay for $120 or less), there's really no reason to pick anything else.
If you want to try Nvidia... good luck. The only experience I have with PPC Nvidia is with the iMac G5's FX Go5200 (which, granted, is an already awful video card gimped even further in the name of thermals), and it's a terrible experience, unable to smoothly perform with even a single program on the desktop even with IceWM. IIRC, the 20 series and up (and who knows how many generations prior) is only able to function as a math coprocessor as of late.
I'm looking forward to something based on Libre SOC. Especially if it gives more BE graphics options in the future.
