Add the fact that you will be using fewer physical servers for the same workloads, and your proposed virtualized infrastructure will look very, very good compared to the existing one. If the new hardware is sufficiently powerful, you may be able to run many logical servers on each physical unit.
Unfortunately, determining how many virtual servers will fit on a physical host is never an exact science. There are tools that can help, though, and some server consolidation tools will allow you to specify the make and model of your current and planned hardware, and will monitor your existing infrastructure for a period of time.
Armed with all that data, you can run reports that show exactly how many virtualization hosts you'll need, what type, and your expected ratio of virtual servers to physical hosts. Some will even calculate the expected power consumption and cooling capacity for the new infrastructure. Investigate the options available from EMC VMware, Microsoft, and others in order to get the most accurate data before you leap into any virtualization project.
But again, don't oversell. It's important for everyone to realize that reducing the number of physical servers does not mean reducing logical servers -- and does not necessarily lead to reducing IT staff. In fact, it's generally beneficial to hire a competent consultant to help plan any virtualization endeavor. Although the basic concepts are simple, the planning, design, and implementation stages can be quite tricky without proper knowledge and experience.
Train before you fire it up
It's also important to take into account training for existing staff. Virtualizing an existing IT infrastructure means changing the structural foundation of the entire computing platform; in a sense, you're collecting many eggs into a few baskets. It's vitally important that IT admins are well versed in managing this infrastructure when it goes live, as virtualization introduces a number of hazards that must be avoided.
If at all possible, make sure your staff is trained before you embark on a full-blown virtualization implementation. Your chosen vendor should provide many options for specific training, or online classes at the very least.
In addition, take advantage of the evaluation periods that many virtualization platforms offer. For example, VMware's enterprise framework can be downloaded, installed, and run for 60 days without purchase, and that time can prove invaluable to familiarize admins with the tools and function of the proposed environment. There's no substitute for this type of hands-on experience.
Don't make the rookie mistake, however, of letting your sandbox training implementation turn into your production platform. When it's time to fire up a production virtualization foundation for the first time, make sure it's with a clean install of all components, not a migration from a training tool.
It's also essential to ensure that training isn't limited to the software. Hardware considerations are crucial to a virtualization implementation, from the number of Ethernet interfaces, to CPU choices, RAM counts, local and shared storage -- the whole works. It's vitally important that your admins are well versed in the day-to-day operation and functions of supporting tools like SAN array management interfaces, Ethernet, or Fibre Channel switches. In a virtualized environment, a mistake that affects a single port on a single server can affect all the virtual servers running on that particular host.
Out with the old
One major benefit of embarking on a virtualization project is that it gives IT the opportunity to jettison old hardware and old frameworks. There's never a better time to inspect the whole infrastructure and identify components that have fallen through the cracks, aren't necessary anymore, or can be folded into other tools or projects.