Best place to start is the hardware of the machines you plan to migrate. Ensure the processors / RAM are sufficient to handle Win7.
Also, determine if you'll need/want the 64-bit OS as opposed to the 32-bit version. The 64-bit OS allows for support of more RAM in the device, and newer / updated programs - 32 and 64-bit, will work smoothly on a 64-bit OS install.
Check your gear, learn what requirements you have, find out what applications you will need to support, and plan ahead for future hardware/application updates...
To migrate from Windows XP to Windows 7.... First u need to move files and data of XP computer(s) before going to install Windows 7. It is better to download a free small toll called <a href="http://windows.microsoft.com/en-US/windows7/products/features/windows-easy-transfer">Windows Easy Transfer</a>, it helps to generate a report the lists of the programs that used earlier in XP.
Remember, this tool may run once before installation of Windows 7, to move files and settings to another location (like removable disk / other spare computer). And again, can use this tool to move files and settings back (from this location like removable disk) to Windows 7 computer after installation.
No need to inspect manually the hardware / devices etc. It is possible to find those compatibility with <a href="http://windows.microsoft.com/en-US/windows/downloads/upgrade-advisor">Windows 7 Upgrade Advisor</a> a free and of course very strong tool. Not only that, it also suggests for the hardware compatibilities for Windows 7.
It is necessary, after installing Win 7 & running Windows Easy Transfer tool, check for new or updated drivers.
Note: U can do the whole process on a test computer (if u have) and after go for the real implementation. U may use <a href="http://www.microsoft.com/downloads/en/details.aspx?FamilyID=816474DB-F854-4AB2-AB59-6C6124CC5053&displaylang=en">Microsoft Deployment Toolkit</a> for more information !