jesus, 400 MHz, that thing has more power than my linux router/server!
Ive heard good things about the ipaq, the only problem I had with them is that they use microsoft...
I have used the Garmin.. er.. I forget the model at the moment, but its basically a palm based pda with built in gps. The next model in development will have bluetooth built in and the capability to go 802.11a/b, with the expansion slot.
Id highly suggest it as well, although its too late for my brain to remember links nor model numbers...