Is it possible to backup a ROM before installing a custom one? Can I backup my current ROM (Samsung modifications plus a few telco specific applications installed) before installing a custom ROM?
The best backup you can do is a Nandroid backup. I didn't know what it was for a long time, but it works like magic. I'll include instructions, but note that i have an EVO, so they might vary a little.


Reboot into Recovery
This is different for most phones, for example, the EVO you reset while holding the `Volume Down` button. Or, since you have a rooted phone, you might be able to get to it by holding down the `Power` button, then selecting `Reboot/Recovery`.

Once in this menu, select `Backup/Restore`

Choose `Backup`, or a similarly named option


Flash your new rom or whatever, and if you are not pleased, simply follow the steps back to recovery and do a Restore. This will reset your phone to it's exact settings from before, down to which widgets you had on your homescreen and what settings you had enabled and disabled.