I'm going to be picking up a copy of FSX this weekend. I was wondering if it's possible to have both FS9 and FSX installed on the same machine. I want to keep FS9, until I can get all my addon planes and scenery updated for FSX. Thanks.

Yes.

Be advised, though, that if you decide to un-install FSX later on, you'll get some small problems... you'll have to re-accept the FS9 EULA, some of your add-ons will need to be re-installed (like AES and Active Camera) and various other little niggles.- Alex