With that card, you are into two troubled waters (for FSX) at the same time. ATIs may give you less fps than the Nvidia things and you are also using a low bandwith/low GPU model.

If you have already switched the shaders (which should help that card), stayed low on the AA side (app. controlled is low, you are right) and also have the cloud density and draw distance in FSX turned down, there won't be much to improve then.  :-/

If you still struggle, maybe put a nowadays cheap GTX460 1GB on your wishlist. This will also improve other games as it's a relatively capable performance card. Cards like HD6870 are also good, if you want to stay at the AMD/ATI side.