If you are using Vista or Win7 you may want to point the installers away from the default install location. Windows adds extra protection to the Program Files folders that can make customizing your FS a bit frustrating. Better to create new folders, C:\FSX and/or C:\FS9, and point the installers there.
The biggest issue with running both sims on one computer only pops up if you uninstall FSX from a computer that has FS9 on it. The FSX uninstaller will torch the registry entry for FS9 and there is a great tool to fix that, you just need to be aware of it.
WWOD---What Would Opa Do? Farewell, my freind (sp)
Never argue with idiots.
They drag you down to their level and beat you with experience