Guys, I am asking this because one of my friends is having downgraded performance with P3D despite having a GTX 680 .. So any card below 780 won't do?

Umm...I have a 660ti and I'm more than happy with my performance. Can run everything but shadows on max.

I used a GTX 680 in P3D2 and good good results as long as I was careful about sliders.  Shadows could be a challenge and so could volumetric fog in heavy cloud cover.  I've since upgraded to a 780ti and it does much better with fog and shadows - but I still have to keep autogen at normal or dense for smooth performance, and I turn off terrain shadow casting (receive is OK).  Still experimenting with cloud shadow settings.  There's no magic card available that will run everything smoothly when maxxed out.  It's still about tradeoffs.  So if you're willing to trade, a 680 should be fine.  

I have a 680 and it's fast enough to run with cloud, terrain and cockpit shadows on @ medium settings.

 

I get 20-30 fps in dense urban environments like Vancouver, and 40-60 fps everywhere else (with ORBX scenery, REX4, and OPUS weather).

Umm...I have a 660ti and I'm more than happy with my performance. Can run everything but shadows on max.

Same here!

I have a 680 4GB overclocked and am happy with performance. 

Ok.. this is on a 660 GTX... so... I'd say a 680 can run P3D.

 

Running a 660ti as well with a very old cpu. Sliders on medium but smooth as butter. Still looks great!

 

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I have a GTX 680 and runs P3d 2.2 quite nicely. But it will get pegged at 99% use, if I throw too many demanding settings at it. The GPU in my case, is the bottleneck, as my CPU use is rarely above 60%. but then again, I have dual Xeons and P3d uses all 12 physical cores. I'm just hanging out to see what the high end nVidia Maxwell video cards look like.