Would you be interested in a governor with a performance guarantee?
If so you should check out the MicroMiser beta available here:
http://miserware.com/products_micromiser.html
It is much more sophisticated than the simple threshold based solutions.
Mat
If you wanted your VM to have an IP and appear as if it is a real machine on the network many people used to have to follow the 100 odd lines of documentation here: https://help.ubuntu.com/community/VirtualBox#Networking
Now they can just start it and it works out of the box.
http://www.bletchleypark.org.uk/
Here is a Mac they had in the 8-bit room:
http://www.matthewgrove.co.uk/personal/moblog/view/2008-11-15/resized_15112008531.jpg
I tell them to turn to the study of mathematics, for it is only there that they might escape the lusts of the flesh. -- Thomas Mann, "The Magic Mountain"