Capacity planning ends up being as important after you begin virtualizing workloads as it was when virtualizing your first. What often happens is no one looks at the capacity on an ongoing basis and deploys more and more VMs and then gets into the situation you're in.
If you're working with production workloads here, I'd recommend looking into VMware's vCenter CapacityIQ product. It is designed to do exact as you describe and may be exactly what you're looking for.
http://www.vmware.com/products/vcenter-capacityiq/overview.html
In addition, if you're looking for a free tool to do an evaluation of the current state then take a look at VKernel's free tools. The tools CapacityView and Capacity Modeler are free and can be used to give you a snapshot of what your environment looks like. They also make a paid product called Capacity Manager that competes with VMware's CapacityIQ product.
http://www.vkernel.com/products/capacity-modeler/overview
http://www.vkernel.com/products/capacity-view
My advice would be to evaluate the products and figure out which fits your needs. Free products are great for one time analysis or "on demand" type analysis, but products like CapacityIQ are better for full time capacity planning.
Matt
http://www.thelowercasew.com
Matt | http://www.thelowercasew.com | @mattliebowitz
