Yes, the two can coexist peacefully. The programs themselves get stored into 2 seperate directories. I believe that when you install a later version it asks if you want to import the settings from the previous install.
sure you can have multiple Netbeans IDE installed on the same machine. even you can run them toghether as they use seperate home directory.
to find out more about netbeans home directories take a look at your user home dir /.netbeans/ you will find a 5.0 and a 5.5beta2 there. in windows users home directory are located at : e.g : c:\Documents and Settings\YOUR_LOGIN_NAME