How do I remove GarageBand? I'm nearly out of space and ~/Library/Application Support/GarageBand is taking up 1.5 GB and GarageBand.app is taking up 381 MB of my hard disk space. 

Can I simply remove the app itself and this folder to remove it from my system?
Yup, if you no longer want it on your system, those are the two biggest offenders in terms of size. To be completely thorough, have a look through ~/Library/Preferences/ for the .plist file and trash that too.

If you're on a purge, it may be worth looking at iMovie/iDVD too if you don't use those.