I recently went through this process with P to V on some hardware as well as migrating an old vSphere environment into the mix, what a fun project.
You can use a tool like live optics (personal edition) to gather information from existing servers and environment. Depending on the size of your environment this could also just be done by hand and thrown into a spreadsheet, mainly concerning CPU/cores, memory, and storage per VM.
As a forewarning if you introduce hardware vendors they are going to recommend at least 3x or 4x your current environment without knowing additional details. An important part of this project is knowing your environment and your expected growth. If you don't foresee any growth then that will look totally different then if you plan on adding additional workloads and storage need. You'll need to figure out what type of lifespan you expect out of the new hardware, and where will you be at the end of that lifespan, (this is assuming you are planning on performing routine hardware upgrades and additions.) Along with growth, in your calculations consider what level of redundancy and HA you would like to prepare for.
Also keep in mind as you introduce your virtual environment you may be adding additional servers for management, plugins, etc. In a small environment vCenter HA can eat up some resources. Sometimes I feel like half of the VMs I'm running are just to keep my cluster running smoothly.
Let me know if this helps any or if you have additional question.
