The notice Connectix sent excluded any mention of requirements. The delivered package definitively says 500Mhz requisite. I have a G4/400 and will be needing VPC early in the new year. So I'd like to borrow from anyone who has upgraded VPC to 6 and is using a G4/400. I hate having to fire up OS9 everytime for VPC 5 but if VPC absolutely positively needs the power, it looks like I'll be staying with VPC 5 for a little while longer.