When plugged in via USB, your computer should see two removable drives. One is the 2GB internal memory "card", the other is the card in the slot (if present). You won't be able to directly access the 256MB system drive.

Organize files however you wish on the cards. Media players should access them easily. The cards will mounted under /media on the N810, but some software, such as Canola will hide such details.

As for a decent music player, I'm looking for one too.