Yep, a Nandroid backup is an exact copy of the state your phone was in at the time you made that particular backup. I always make a Nandroid right before flashing a new ROM. That way if I don't like it, I just do a restore and everything is restored as if nothing had ever changed.
BTW, are you going to CELB Froyo 1.5 (2.2), or CELB 2.8 (2.1)? If it's the latter, be sure to partition your SD card first if you haven't done so yet (and make sure to backup your SD card to your computer before partitioning, as this erases everything - then just copy the files back to the SD card after partitioning). If it's the Froyo version, you need to reformat the SD card I believe, and get rid of the partition (2.2 won't work on a partitioned SD card).
I'm using CELB 2.8 (the 2.1 version) and I love it.