You can place the Logic 7 and 8 applications in the Trash to delete them.
The reasons to keep them would be to ensure you can revert to an earlier version if needed. Might be an idea to keep them somewhere on CD/DVD just in case you open a project created in 7 that isn't working quite right in Logic 9.
To be honest... it hasn't happened to me and I've deleted them both the 7 and 8 applications ;-)
As you perform software updates for Logic 9, you'll notice extra versions of Logic appearing in the Applications folder. These will allow you to rever to earlier version of Logic (e.g. Logic 9.0.2 rather than 9.1.1) if you wish.
It's gives you a fail safe way to test out the updated version...