Don't buy Virtual PC.
Virtual PC costs $249. You can buy an actual bare-bones PC from several places for about $250. (See Walmart)
Use Microsoft's free and excellent "Remote Desktop Connection Client for Mac" to remotely control the PC from your Mac across ethernet. This way you don't even need a keyboard or monitor attached to the PC.
You'll end up with a faster, better experience than VPC. This is what I do to program my Garmin eTrex and SPIII.
Cheers,
-Jason
(PS: I just emailed Garmin asking them to support the Mac platform. They wrote back thanking me for the suggestion. I would encourage all Mac users to do the same)
