Well, in my case *not meaning to brag* I have most of my settings set to High, as I have done alot of daring moves to make sure my comp. can handle the game. Only Javier knows of my secret!
Although, I do have autogen and water effects very low, as I think it's quite nice to have sparkly water, but I'd rather have a fast game rather than a slow game, so I'd reccomend about middle on most things
I also believe Aircraft should be quite low, and regular traffic quite low, as that's a BIG frame rate killer, however, I have Scenery, and Airport traffic Ultra High. I think that you'd get the best out of a poqwerfull system, which can just hold FSX!