The process for migrating computers in this manner broadly speaking involves,
- A personality capture
- An image deployment
- A personality deployment
What is important to note is that any software you have on the XP machine which is not in your Windows 7 image will have to be re-installed. A migration package like PC Transplant, concerns itself (on the application side) with settings but not the applications themselves. So you need to inventory the XP machines to make sure that when you migrate you have a Windows 7 machine prepared with the correct software base.
The personality capture step gathers all the settings on the XP machine which you want to preserve. This creates a single binary which you can then deploy to the Windows 7 machine to transfer them. In GSS3, there is a samples folder which contains some example jobs which use PC transplant to capture things like printers, desktop and application settings. More details on using PC Transplant can be found here,
https://support.symantec.com/en_US/article.DOC1108.html
There is also a terrific series here on Symantec Connect by BlairThomas where he covers migrating to Windows 7 in great detail,
http://www.symantec.com/connect/articles/windows-7-migration-introduction
My advice though is if it's really just a few machines, you'd likely be faster just deploying the new machine and copying data manually. Just sayin' ;-)