As a starting point, neither. Keep going older until the next step can't do what you want to do.

I don't think there would be much practical difference between 7xx versus 9xx series. Depending on the system, it will likely be CPU bound. My own GTX 980Ti doesn't break a sweat running XP era games.

And Far Cry works fine in my experience with that 980 Ti. The only issue I had was having to limit the frame rate, otherwise it tries to max out FPS at the main menu (hundreds of frames a second).

If you dual boot with a more modern OS or want to keep that option open for the future, I would go with the 9XX series.

Yeah I'll just go 9xx, thanks. I think I was expecting a similar situation like with 98 and the later cards losing table fog and palleted textures but for XP it doesn't seem as bad (no doubt there are some edge cases here and there). I also need to look into which version of XP to use, initially I was just going to go with XP SP3 but then I remember some discussion on people preferring SP2 or even earlier. Will have to experiment.