Admin vs. Non-Admin and impact

We are a company of 1500 employees with approx 10 office locations and one Service Desk located at our headquarters. We are moving from XP to Windows 7 on our client machines. At present, our employees have admin rights on their machines...not a great scenario. With the transfer to Windows 7 we would like to begin employing non-admin rights. We are concerned, however, how this change will impact our Service Desk. Upper management would like stats on how other companies "run". How many employees are there? What is the Service Desk technician per employee ratio? What are the responsibilities of the Service Desk...do they build and decommission computers as well? What OS is being used? Do users have admin rights? Etc. If you would take 10 mins to tell me a bit about the environment you work in it would be much appreciated!