Virtualization vendors offer several tools that can forecast what hardware you'll need to move a physical infrastructure into the virtual realm. These tools, such as VMware's Capacity Planner, require some setup and configuration, but can provide a wealth of extremely useful information before you spend a dime on production hardware.

These tools employ constant performance profiling to gauge the resources that each server on your network consumes, generally over a period of time from 30 to 60 days. They see the peak utilization of CPU, RAM, disk, and network I/O resources, and mix all of that data together to produce a guide to the CPU, RAM, storage, and network requirements you'll need to shift the infrastructure into the virtual world. In some cases you can even define the brand and model of the servers you're considering, and the tool will tell you how many you need. Playing with the numbers here can save you plenty of money down the road.

Based on the results from your lab testing and capacity planning, you should have a good idea as to what resources each of your physical host servers will require in production--at least to a point.