The option I went for was to buy TomTom complete with GPS receiver and cables. This allows me to plug the GPS into the iPAQ and power both from an accessory socket. I can use it this way on the bike and in the car.
On the bike I use a RAM mount to connect it to handlebars and a waterproof Otter case to house the iPAQ. The GPS sits on top of the TripMaster (Africa Twin) and the power comes from a socket I wired up just in front of the instruments.
As you already have a GPS you could see if you can get a cable to connect the Garmin to the iPAQ. Bear in mind you will also need to power them both as well.
I've used the system a number of times now travelling around the UK and it works very well. European maps are also available now (at extra cost).
If you want any pictures let me know and I'll e-mail them to you.