First off DO NOT try to upgrade this phone to WM5 or WM6. It simply will not work.
The best you can do is to update the firmware. I bricked my PDA so many times I do not remember what I used to bring it back to life.
Check xda-developers.com for updates.
Bluetooth Headphones
I recently got a pair of Jabra BT620s Wireless Streo Headphones but to pair them up you need to patch the PDA with this: PDA2K_BroadcomCorp_BTUpdate.zip
You now have a the ability to use Bluetooth Wireless Headphones with your PPC6600.