Sure you can delete them. Just drag them to the trash. Garageband leaves a few files lying around the place so maybe do a quick search for the two apps (AppleWorks and Garageband) in Spotlight to get rid of absolutely
everything.
Then, if ever you need to reinstall them, you can just get them from your Restore DVD/CD but you have to use a special program to be able to read the package contents. It's called
Pacifist.