I have used both the HD 7970 and a GTX 680 for FSX. I actually found the 7970 to be smoother, especially with hi levels of FSAA with Super Sampling. I think the 3GB of VRAM helped along with the higher bandwidth. Though I do use 2560x1600 resolution. Having said that FSX is not my main sim and the HD 7970 was a reference version with a very loud fan. So I sold it and kept the GTX 680, much quieter.
Intel i7 3930K @ 4.3Ghz 16GB RAM 2 x EVGA GeForce 680GTX Superclocked Signatures in SLI ----------------------- Using the full 1680x1050 settings: Min: 57 Max: 130 Avg: 83.415 Using Playable 1680x1050 settings: Min: 72 Max: 185 Avg: 122.702 ------------------- I usually play Rfactor2 at 5960x1080 (bezel corrected) on 3 x BenQ XL2420 Monitors, in Nvidia 3D Surround.