External video card for iMac, worth the game?

Question

NVIDIA

Good afternoon.
There was a need to increase the graphics power of the iMac 27 (new games do not pull).
The processor and RAM allow.
Of the possible options, without replacing the computer, only an external video card.
From this a number of questions arise.
1. Which adapter should I use if I need an AMD Radeon 7990 sized graphics card? Not every one of the found adapters could accommodate a similar one.
2. I suppose that you need to connect via thunderbolt? also with external power?
3. How well will it work in OS X or Windows dancing with a tambourine? curve setting? or just connect and switch something in the application from Nvidia or Catalyst will be enough?
4. Which video card is better for gaming? Nvidia or AMD?
5. Will AMD have problems under OS X? Firewood, etc.
6. Performance? will it be possible to use the video card to its full capacity?
7. How inadequate is the solution on a scale from 1 to 5, where 1 is complete nonsense, 5 is it normal?
Maybe there are some other nuances?
PS The idea is not mine, iMac too. I'm just passing on the question.
Thanks in advance for the replies.