Exchange Server 2003/5.5

I am presently running Exchange Server 5.5 on a Windows 2000 server. I am ready to install Exchange Server 2003 on Windows 2003. My goal is to put the Exchange Server 2003 online to test it while still using Exchange Server 5.5. After everything is good I am going to switch to the Exchange Server 2003. I want a backup plan in place so I believe I read somewhere that you can create multiple MX records with a priority to which server the mail goes to. If some reason it doesn’t reach the new server, it will be delivered to the old server.

QUESTION: Is this correct. Are there any suggestions to what I am doing other than good luck.