PlateSpin Recon is owned by Novell, and they have a great product on their hands. With PlateSpin Recon you can analyze all the servers in your environment and track them over an extended period of time. Then after you have collected the data you can go through planning stages with the program to determine what type of hardware and how much hardware you will need to virtualize to.
A product like this can save you tons of money. Novell offers a free trial of the software on their web site. I recently had a chance to use it and have to say that I love the product. It provides valuable statistical analysis data during huge consolidation projects and really takes the guesswork out of all of it.
This will allow you to make sound decisions on factual data when placing your virtual machines. You can find out more about this great product here.