Suggest some tool for user migration

Hi
We are ready to apply a new AD domain construction with a number of branches through the country for association. Nearly all of the branches set only Active Directories, a few – just workgroups. AD uses Windows 2003 as well as Windows 2008, but users’ PCs run with Windows XP along with Windows 7 OS while the new AD will use on Windows 2008.
Could you suggest some tool for user migration?
As I realized, similar migration can be applied for Exchange servers/accounts. Now a large amount of the branches access headquarters’ mail server (Exchange) out of the country; one or two branches access only local mail servers.
Would it be great to automate migration method and optimize traffic, if possible? Server will be replaced in 2007 where Mail client is MS Outlook 2007,
Please suggest some migration device for Exchange. Thanks in advance.
