Added an 8gb sd card, which I then formatted. How do I 'use' it? Can I control whether music that I add to the DI is stored in the sd card as opposed to the internal memory? How about when I take pictures/videos, etc.??
Download Astro from the Market (it's free); will help you manage your files on your phone.
You can also store ringtones and notifications on your card. Set up file folders with those names and put the files there. Also, make sure your email attachments storage is set to store to your card. HTH!