I haven't tested Virtual PC yet but I swear by VMWare (workstation and GSX). I've got VMWare workstation on my laptop and run 4 different OSes under it (98, Red Hat EL3.0, Win2K Server and Win2K3 server) and I have no problems. I don'r run them concurrently as I've only got 512Meg RAM.
I've got GSX server running on a server upstairs. On that box I have 2 Win2K Server, 1 Win2K3 Server and one RH EL3 server running concurrently. These are test and development boxes and work like a charm. Using VMWare I dropped from 4 2U servers to 1 1U server. The 2U servers then went on to become production servers. The cost saving from the server reuse paid twice over for the cost of VMWare. Factor in the snap-shotting capability and the ability to move VMs from GSX to my machine and back and it more than pays for itself.
VM's rule!