Yes, you should be able to run mapping software and Excel at the same time, providing you have the memory to handle both. For more detailed information on GPS, one of the bette sites is
GPSPassion.com. They have reviews and discussion forums for all kinds of GPS/PPC combinations.
Mapopolis works with any GPS that outputs NMEA data. I have a Magellan that I hope to interface with a serial cable (when it arrives). I'll report back on the success. My rationale for having a separate GPS unit is that PPC's aren't really designed for exterior work, and I wanted a GPS unit that was more rugged for that kind of use. I use the Magellan with Mapopolis for driving directions and guidance, then just the Magellan for the outside work. Nice combination.