ROM, Flashing, Odin, Stock, etc.

I know I will be kicking myself for asking this and I somewhat know an answer but I would like to hear it from the professionals.  "To flash a new or different ROM do I ALWAYS need to go back to stock?"  Meaning use ODIN;  press "START" then later use "MASTER CLEAR?"  I would be so appreciative if someone or anyone can clarify this for me.

p.s. What if I just want to update a custom ROM?  i.e. Cognition 3.04 to Cognition 4?

So really no need to "MASTER CLEAR"....

don't odin back to stock unless you have to because the odin-back-to-stock re-flashes your sbl.bin (basically your boot files, think BIOS), and if you accidentally disconnect during that process, you will end up with a real brick.

Hey guys,
Sam Cappy, Stock I897UCKJ3_withBL through Odin 1 click, Fugu rooted running boog_KJ3_kernel3 right now.
Noob question that I can't seem to find the answer for in search.
I am looking for a way to save a whole ROM as it is configured currently, meaning all phone settings with every app and their settings including the root, from the phone to either an external SD card or directly to my PC to reflash at a later date through Odin3 1.8x.
I have a current TiBU saved to the external SD and a copy on the PC but I don't believe this will do what I am wanting. I have Rom Manager/Clockwork Mod but the Flash Clockwork Recovery function doesn't show the Captivate SGH i897 only the i9000 for Galaxy S. I have also heard the the Save Current Rom function is pretty usless anyway.
I know how to flash via Odin3 1.8x or a simple 1 click but have not done it with CWM and have no idea how to image the current ROM as set now.

TYIA
Bud