If it hadn't had been for this forum FSX would have been a dud. With all the tweaks that have been posted on here i now have fsx running at 25fps on ultra high high settings. I know this has been mentioned but for those interested i'll just mention the few that i think are most important.
FIBER_FRAME_TIME_FRACTION=0.25
TEXTURE_BANDWIDTH_MULT=400
WideViewAspect=True
TERRAIN_MAX_AUTOGEN_TREES_PER_CELL=2300
TERRAIN_MAX_AUTOGEN_BUILDINGS_PER_CELL=1300
Also do not use the antiliasing option in the game itself, if you're video card control panel has the option of setting antiliasing do it there!
Last of all be patient, if you carefully read all the messages posted in here you can get FSX to work a treat and in my opinion it is better than FS9.
Don't just install it, run it and then bag the crap out of it, you need to play with it for at least a couple of hours to find the settings that suit your machine.