best PDA to do the following?

play MP3's

large color display for Garmin maps

take generic expandable RAM.

Bluetooth

standard Cardbus slot so I can plug in one of the new 3G aircards for web
browsing -- is this available on any of them?

affordable interchaneable LiIon batteries.

clamshell design would be nice.

I have no particular OS requirements, though I will hotsync to an XP
laptop.