Is it possible to make a backup of the flash, just in case I screw something up while messing with my N800?

I have access to a Linux computer, and I have Diablo with latest SSU update.

yes! Use penguinbait's How to clone to SD guide, and you will have a copy of your OS in the SD (with all your programs and customizations) in fact, let the flash be your backup, because if you run it from SD, you get a lot more room to install programs. http://tablethacker.com/wp/?p=151&lang=en-us

The best may be:
Clone to internal SD, giving plenty of room to install programs.
Occassionaly clone the internal SD to an external SD, as a safety backup.

Yeah, I've been thinking about doing that, but I heard that doing that causes the power to be used more because the device cant shut down the internal MMC.

Also, I use my internal card for mainly music. Would I still be able to store ~500-700mb of music? I have a 2gb card.

well im not sure how much it takes from the card, but what's left its yours to use. Not sure about the power drain, I never noticed it when usind the sd card.
if you want to just backup, you can do the clone then use the flash. and deactivate the boot menu till you need it again.

That sounds like a good idea... is there a way to restore the clone back to flash?