I use X-Plane all of the time with my i7 4 ghz Retina iMac. I'm running the 290X and am running the game at many of the very highest settings - stand by I'll show you a screen shot of what this "base" card can handle shortly.....
Okay - just because I'm crazy I decided to film me flying the Twin Beech with one hand on the joystick, occasionally bumping the throttle, filming with one hand and using the mouse with another so you can see framerate performance and what I was doing (flying like a maniac basically on a very windy day and realweather reflected it).
The Joystick and throttle are the Saitek X-55 Rhino H.O.T.A.S system - works quite well for X-Plane and is about the closest thing I'm going to get to flying short of getting into an aircraft.