I don't know the answer to your question about the folder for attachments, but I have a suggestion for you:
do a search for storing email on the storage card with wm6. The registry entries also will create a folder on the storage card to store the attachments. I believe you could do just the registry entry for the attachments (and leave the emails themselves in the main storage area).
I've done both registry mods, and have had no problems. The folder on the storage card contains many files with names such as att12351.dat.
edit: here is the link to the storage card thread:
http://www.everythingq.com/forum/mot...ard-10901.html
Jason