Rom manager backs up everything in one file. You can't use it (to my knowlege) to back up data in apps. My Backup does that. Rom manager makes.one big backup of everything (called a nandroid) including the old rom so if you flash a new rom the go to rom manager and choose restore and select the file you saved (name of it is the date it was saved) it will restore everything back the way it was before...you lose the new rom and put back the old one. It takes a long time to restore everything so just leave it alone while it restores (hopefully) back to the original rom. Then you can download my backup, backup apps and data, flash the new rom, restore data and any apps that don't download automatically from my backup.
There are directions (better than mine) under rooting, roms and hacks in a sticky.