The Windows XP pro upgrade on Tech Soup has the following restrictions for qualifying products:
This is an upgrade product. To install this operating system, you must have a valid license that is not in use on another computer for one of these operating systems: Windows 98, Windows 98 SE, Windows Me, Windows NT Workstation 4.0, Windows 2000 Professional, or Windows XP Home Edition.
Under software assurance you can then upgrade to Windows 7 Enterprise ( I checked the last XP license donations from Tech Soup on eopen that we received)
If your vista home came with downgrade rights to XP home, In my opinion that is the path I would use to get to Win7
An overriding question: What are the business reasons to get to Windows 7? What features do you desperately need that you must upgrade from XP or Vista to WIN 7, and it can't wait until you do a hardware technology turn?
For us, I plan to stay on XP with our current hardware, use donated, refurbished or off lease XP pc's for any task workstations for the next year. I will either upgrade my personal laptop to WIN 7 on a new hard drive, or pick up a Win 7 desktop to take a look at it in the coming months, but will not plan any Win 7 deployment unless there was a funding windfall for new PCs.
Dave