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Thin computers are nice and all, for what they're worth, but if they cost $450 apiece like the higher-end HP's, you may as well just buy regular HP desktops.
We use a variety of tools when planning rollouts of new operating systems. As XP is pretty old - 10 years - it's probably time for a change. I recommend checking out this Microsoft website which can help you prepare for your migration.
How many PC's are you upgrading? For the cost of licensing those machines, it may be worth it to your organization to consider retiring them, or at least some of them, and using thin clients as a replacement. You would set up a server running that would host the Windows environment, with all your corporate-approved applications pre-installed, then people login on their thin clients and are presented with the Windows 7 environment on their desktop. We're using these in our branch offices (HP T5570) and it makes management a whole lot easier and less time consuming, because there are fewer parts that go bad.
First, prepare a hardware inventory listing all the pc's (laptops and desktops) and their cpu, speed, RAM, and hard drive size vs. available size. Windows 7 has minimum recommended specs, and any machines that do not meet the spec should be retired. Microsoft says it will run on a 1ghz cpu, but trust me, you want at least 2ghz; Microsoft says 1gb RAM is fine, but you want at least 2gb, 4gb if possible. Older 32-bit only pc's cannot run 4gb RAM so those should be retired. Windows XP cannot be upgraded to Windows 7; it must be installed fresh. You'll need 16gb-20gb of hard disk space to fit Windows 7, but if you're upgrading from Windows XP, you'll need to erase those drives; it may cost more to buy replacement hard drives, but it will save you hours of hard disk formatting if you swap out old drives and install fresh on a new hard disk. Although it's possible to roll out Windows 7 a number of ways, usually installing from the DVD works best. Also you may wish to prepare an external USB drive or flash RAM with the SP1 update and have that ready for running immediately after you install Windows 7. That's faster than downloading it individually for each pc.