Dennis,
I've been using Virtual PC 6/Windows 2000 with a G4-466 for the past year (and versions 2, 3 and 4 before that). This version doesn't run on the G5 computers, but does an adequate job with Excel VBA, Internet Explorer. Adequate means slow, but workable.

I have read that Virtual PC 7 is a little faster in some aspects (and of course is compatible with G5 computers). It also restores the ability to print files and it supposedly doesn't crash when Windows 2000 wants to do a live update (both of these problems resulted from the version 6.1 upgrade that Microsoft released after having bought the package from Connectix). The increased hardware specs are probably due to the use of Windows XP Pro as the default operating system. Windows 2000 will definitely be faster.

Tuning Virtual PC for speed is a real art. Turning off Norton AntiVirus and allocating only 256 MB of RAM definitely help.

If you haven't discovered it, MacWindows is a pretty good resource http://www.macwindows.com/VPC7.html
Brad