Before Intel Mac's, which have the ability to run Windows at native (Bootcamp) or near-native (VMWare Fusion) CPU speeds, I tried using Virtual PC on my Powerbook G4 laptop to run Garmin Mapsource. Unfortunately, it performed so slowly that it was too painful to use.

At the time, I was using a Garmin GPS V, which connected with a serial cable. The serial data rate with the Keyspan USB-to-serial adapter I was using was so slow through virtualization that Mapsource kept timing out and losing connection with the GPS.

However, if your Mac is a very fast G5 model, VirtualPC could work, but don't expect great performance. WindowsXP SP2 will feel very slow, like a 500MHz P4 at best.