I have two i.Trek serial GPS units for sale that work great with the Treo 600. They come with the car charger and combo serial cable / Treo 600 cable. (not including Mapopolis software or vent mount)

Details can be found here http://store.yahoo.com/semsons-inc/itmousgpsrec.html

I'll sell them for $65 each shipped. (almost 1/2 price)