Summary of pros and cons of server virtualisation
Server virtualisation has been one of the major trends in IT for the last decade and with somewhere between 50-65 % (figures vary) of X86 servers now virtualised across the globe it has proven itself a reliable and popular extension if not full replacement for X86 native hardware.
Server virtualisation should save you money but to answer the question of how much is not so simple it really depends on the number of servers you have and what resources they require. A number of tools exist to assess the current environment and to help gain more insight into how your virtual server environment might look.
From Microsoft there is Microsoft Planning and Assessment Tool Kit ( MAP) which is free and client installable and from VMware there is Capacity Planner a partner installable tool, 3rd party tools also exist.
Apart from the obvious cost saving of having fewer servers taking up less space and less power, the benefits of easier and faster backups and disaster recovery as well as lower management and time to deliver a server/service need to be taken into account as these will have a longer term effect rather than just reducing the server numbers.
Not all applications/operating systems with license/support agreements permit virtualisation, in all instances you should check the license and support contract for anything before you virtualise it. You may find that you can't do that per the agreement.