We're in exactly the same boat looking to migrate from server 2003 to 2008 and from Win XP to Win 7. A solution for deployment we're very keen on is Microsoft System Center Essentials 2010 out this year. Will also provide for zero touch installation which is also extremely useful with so many workstations on site!
Provides a lot more than just deployment though and is a good all rounder for a lot of management tasks which helps to justify the licensing cost