While troubleshooting a CPU performance issue on a workstation class VM, I wondered if there were a way to assign virtual CPU cores to a VM. Check out the following links, the processes documented on them worked great. In this case, after performing the steps therein, my workstation VM believed it had 2-Quad Core Processors.

http://www.vcritical.com/2009/09/use-coreinfo-to-view-vm-core-and-socket-count/

http://www.yellow-bricks.com/2009/06/04/per-processor-licenses-for-your-application/