I'm testing a few configurations of W2008 server as a virtual machine with vmware.
I can change RAM from 1024 to as much as 4096GB and add 3 SCSI virtual disks -- no problem whatsoever. I can even change the VM to use 2 processors instead of one -- normally you wouldn't want to do that on a typical machine but the Host is a 4 processor CPU 12 GB RAM machine. Still no request for Re-activation.
Now if I do something really innocuous like simply add a DVD device to the (ORIGINAL) virtual hardware I get the message " Hardware has SIGNIFICANTLY changed -- Windows must be activated within 3 days.".
I'm not sure what tips the activation mechanism but adding a CD / DVD device IMO is a lot LESS significant than adding a new processor or adding 4 yes FOUR times as much RAM to the initial configuration.
So a tip to guys messing around with VM's -- keep a "Base" activated copy of your VM before changing a lot of the virtual hardware so you can avoid multiple re-activations -- I'm not sure how many MS allow you but trying to explain to people in the activation center that I'm testing Virtual Machines won't be much fun.