Mark has a post on Running SharePoint Portal Server on a Virtual PC. Good post. for reference, the machine i used at the INDA Demo was a 3.6Gz P4 with 1Gb ram, 160Gb hdd and Windows 2003 server as a base OS. The virtual PC was given 512mb ram, 16Gb hdd, and a full network card. this system was (and still is) very stable. i use it for my development and testing of sharepoint, and i still post files to it for internal use.