I have a relatively old but highly flexible GPS setup. I have a Pharos bluetooth GPS receiver about 2" square x 1" thick. I use it with my Palm Treo 650 cell phone/pda and Mapopolis software. This is really portable since I have my cell phone with me all the time anyhow. The Pharos has batteries in it, so I can also use it as a totally handheld system. The Mapopolis software is pretty basic, but it works pretty well, it even has voice prompts on the Treo.
This has been a great help on a couple of trips to Italy and Spain. Navigating the streets in Genoa, Florence and Madrid is a challenge even for the locals!
I also have Microsoft MapPoint on my notebook with Bluetooth, so if Mapopolis does not work, I use my notebook.