Addressing the licensing question
Now that you've done the footwork and have PCs ready to deploy, it's time to think about licensing and activation. In enterprise deployments, it's best to use a volume license and the product activation approach called Volume Activation (VA) introduced with Vista. To use VA, you need either a Multiple Activation Key (MAK) or a Key Management Server (KMS), which requires a KMS key. The mechanics of getting these keys is complicated, so Microsoft has a help page on how to do it.
Note that Microsoft has many types of licenses for Windows and other products. When upgrading to Windows 7, be sure not to confuse the volume license with the Enterprise Agreement, Enterprise Subscription Agreement, or Software Assurance (SA) programs. The two agreements are essentially maintenance agreements that include the SA program, which lets you upgrade to a new version of Windows at any time during your agreement period, in exchange for a per-system annual fee. Given Microsoft's slow OS update schedule and the lack of transferability of SA coverage to new PCs, this insurance-type plan has ended up costing businesses more per desktop than simply purchasing upgrade licenses for old PCs and getting the OS included with new system purchases.
Migrating your PCs to Windows 7
You have probably already heard the news that XP cannot be upgraded in-place to Windows 7, so your applications and data are not migrated. On a single system, that may be frustrating (although you could upgrade to Vista and then upgrade to Windows 7), but larger businesses don't do in-place upgrades anyhow. For corporations, in-place migration from XP is a nonissue, at least for the OS and the apps. Where there is an issue is migrating "personalities" -- the user settings and data -- from the old computer or image to the new one.
To aid that "personality" migration, Microsoft provides the User State Migration Toolkit (USMT) 4.0. It supports two migration scenarios. One is moving the data off a PC before Windows 7 is installed, then moving the data back afterward (called a PC refresh). The second -- and depending on the age of your current desktops, possibly the more important scenario -- is moving the files and settings to a new computer (called a PC replacement). USMT pulls information from the hard drive, the registry, and other Windows data and restores it to the refreshed or replaced system.
The Microsoft Windows Automated Installation Kit (AIK)
Part of working with the USMT involves the use of the Windows Automated Installation Kit (AIK) for Windows 7. This kit includes the USMT; the Windows System Image Manager (SIM), to create unattended XML answer files; the ImageX tool, to capture and apply images; the Deployment Image Service and Management (DISM) tool, to manage your images by adding and removing drivers, language packs, and patches; and the Windows Preinstallation Environment (WinPE 3.0), to create the OS image you want to deploy.