My two cents:
o Time Span - When and how long do you expect it to take.
o List of Tasks - Prepare a list of tasks, softwares (drivers) and hardware (cables, CD-RW) required to take care of the migration.
o Man Power - How much man power will be needed for this migration under the time frame ?
o Fallback plan - What if new machine doesn't run old software properly?
o Communication - Prepare template of emails/letter to the colleagues of concern, asking what they need to backup or cleanup.
o Inventory - Label the machines and save the information into an excel worksheet - hardware contents, maintainance period, vendor etc.
o Define good policy in utilizing the resources - Users often use computer in a "convenient" way, that introduces a lot of unwanted effects - such as data files spreading throughtout the drive.
Hope this helps!