I'm not an expert at any of this.. just sharing what i have learned.
2.1 is great, I don't look back at 1.5 at ALL.
To go to 2.1 root you have everything you need.
the B00IMG zip, as well as the recovery zip, then the all in one root script.
From there load up the eris21official-root.zip
Setup your google account, any other e-mails etc..
then reboot your phone into recovery mode. (Vol up + power)
Go to Backup/Restore > Backup NAND
That's your base point as to what you compare all other roms to.
To go back to it: Just do a Factory Data wipe and a Davlik wipe.
Then Restore the NAND backup.
Every time you try out a new rom, or go back to an old via nandroid backup.. you need to wipe the Factory data and Davlik.
Play around.. you'll get it