Are you running it in DX10 mode? FSX seems to run smoother under DX10 on my Win7/64 box with Nvidia GPU. Other than that there do seem to be some bottlenecks in the FSX code itself as things get jammed up on the CPU side, even on my Q9650 @ 3600 mhz. Very fast RAM is the best help for it but there's no real cure. Almost forgot: bloom and ground shadows will absolutely clobber your FPS with very little visual improvement to show for it. Again, FSX is not properly coded to use the power of the GPU.
