Do a desktop backup - use a 100 mbps connection
Do the desktop back up even if you have been doing a tape backup - thatway if your tape restore fails the desktop backup will give you a plan B.
Read all the bug reports
If you have more than 2G data in the ibays - backup that to a remote share and then delete it from the server and restore later.
remember that contribs used on 5.1.2 may not work on 5.5 and that you will need to install any contibs B4 doing the restore.
We are sticking with 5.1.2 in anticipation of 5.6
rob