I have posted about this in the past, but HP have updated their virtualization sizing tool.  This is a very nice tool that allows you to enter a bunch of data about the physical hardware that you want to consolidate, and then makes recommendations about what hardware you can purchase.  You can go check it out here: http://h71019.www7.hp.com/activeanswers/Secure/281742-0-0-0-121.html

Cheers,
Ben