What should I backup before installing a new ROM?
Jumping between ROMs in the same development series is usually safe without backing up or wiping the data partition (although the cache and Dalvik cache should always be wiped), unless the installation instructions specifically tell you to wipe the data partition (which seems unlikely in this case; if such a drastic change were needed, it shouldn't be a release candidate any more).  I would nevertheless back everything up just in case, both application backup like Titanium Backup and Nandroid backup from the recovery partition, but then I'm paranoid like that.

Jumping to a different series (say, CM6 to CM7, or CM to aosp) you should always back up everything and wipe everything.