NAND
The Pandora has 512mb of internal memory on the NAND and that is used by the firmware. All directories except for what is in /media are located on the NAND (see Basic Linux Guide).
A clean installed unit will have "about 80mb left". Not being careful (especially by installing too many things with OPKG) could mean that you'll run out of room and need to reflash your Pandora.
It is highly recommended to run Ångström or other OS from SD card, by doing that you also avoid wear and tear on the NAND.
Type
df -h
into the terminal to check how much space you have left on the NAND. Type
du -h | more
into the terminal to find cache folders and how many megabytes they are using (then delete what is in them to free up space).
A more advanced approach will be:
du -x /|sort -n|tail -20
which will show you the top 20 in directory sizes on the NAND only.
