If you're looking to increase performance specifically in FSX, be very careful with video card upgrades.
Being a truly free form simulator, it is impossible for a developer to create individual frames for all situations, like a typical game is developed. FSX renders each frame on the fly. The CPU creates the image in real time, then sends the rendering instructions to the GPU. There is a massive buffer in between to smooth any situations where the CPU hasn't provided enough frames or the GPU hasn't had time to draw them. This buffer is incredibly taxing on the CPU. It's a HUGE bucket to fill. It's that way for a reason. If the CPU were to provide the GPU with too many frame instructions, the GPU would be unable to draw the next frame and you have what's called stalling.
Basically, FSX is CPU bound by design. This is especially true since GPU performance has grown by leaps and bounds since FSX was released. CPU performance has increased, but not nearly the increases AMD and Nvidia have had with the GPU. So, as it stands, most of the time your GPU is just hanging out waiting to be fed instructions by the CPU, having drawn the last frame long ago. Of course, you can work the GPU a bit harder by letting it draw a nicer picture with some AA and anisotripic filtering. If it's more frames you seek, it's all on the CPU to send more.
That you're having issues is odd. You have a modern processor. While FSX only utilizes two cores for instructions, your clock speed is pretty good, and that's a plus. A 5770 isn't a slouch either. My 2600K and old GTX260 ran without a hiccup, even in high autogen locations like Seattle. I do keep my autogen elements in check a bit, but details are all maxed.
But the rest of my games are pretty good.
