Portable automobile GPS (Garmin, TomTom, Magellan)

Similar to questions in the roadtrip software planning thread, I'm curious about portable GPS units that are designed for on the road navigation. There are several all-in-one units on the market in the $500-800 range that seem to be quite good, but I'm not sure I'm ready to pay that much for a navigation unit.

I have used a laptop with Delorme Street Atlas USA software and the Delorme Earthmate GPS receiver, but the laptop is bulky, requires a DC/AC inverter for constant power, and just isn't suited to use in a car like the compact GPS units are.

Delorme also makes Street Atlas USA software for handheld PCs and PDAs. Because I already have the Earthmate GPS and the software for handhelds is only $40, I thought a more economical route would be to buy a used PDA with color screen to replace the laptop. The only concern that I have with using a PDA is the amount of memory. I'm afriad that the PDA will not be able to store all of the map data necessary for a long roadtrip.

Do any of you have experience with the handheld version of the Delorme (or other brand) software? What are the limitations compared with the all-in-one units made by Garmin, TomTom, and Magellan? I look forward to hearing your experiences. Thanks!