Microsoft offers a free Windows 7 Upgrade Advisor and that is the best place to begin your upgrade odyssey. Also, here is a link to the official Windows 7 Upgrade and Migration page. You may want to read it before making your upgrade decision.
(See Related: Everything You Need to Know About Windows 7)
Be warned: Vista users get to upgrade while XP users will migrate to the new OS. The difference is important, as you will see.
Windows Vista Users -- If you have survived this long with Windows Vista, you may be in no hurry to upgrade. People who like Vista may actually consider Windows 7 a step backwards in some regards.
The good news is that you can upgrade a Windows Vista machine to Windows 7 with a minimum of difficulty. Also, a machine that runs Vista may run Windows 7 a bit faster.
Windows XP Users -- On the Windows Upgrade Advisor page, Microsoft states: "If your PC can run Windows Vista, it can probably run Windows 7, but it's still a good idea to use the Windows 7 Upgrade Advisor."
That means newer Windows XP machines can migrate to Windows 7, perhaps where you chose not to have Windows Vista preinstalled. However, if you are running a 5-year-old XP machine, I would not expect to upgrade or to have some regrets if you do.
Be warned that there is no easy upgrade option for Windows XP to Windows 7. Here is what Microsoft has to say about it:
"The upgrade option is not available in Windows 7 Setup when installing Windows 7 on a computer running Windows XP. However, you can use Windows Easy Transfer to migrate files and settings from Windows XP to Windows 7 on the same computer.
"To do this, you must first copy files to a removable media, such as an external hard drive or UFD, or to a network share. Next, you will install Windows 7 and then migrate your files back from the removable media onto your computer. When you are finished, you must install your software programs again, but your files and settings will have been copied from Windows XP."
Corporate IT -- I will not presume to tell IT Pros whether to upgrade. I believe many companies will find the new enterprise features of Windows 7 a good reason to switch, though many may wait until the OS has been on the market for a while and the presumed kinks have been worked out.
Small Business -- I treat small businesses much like consumers and, to that extent, I don't recommend upgrading XP machines unless they are fairly new and you don't mind having some people running XP and others running Windows 7.
New Computer Buyers -- This is the very best way to get Windows 7: Preinstalled on a shiny new (and faster) PC. If you have had your XP machine for a while, you will be impressed with performance, features, and the low prices now available. This is how Windows 7 has entered my life.
I am not planning to upgrade my XP or Vista boxes to Windows 7. Not because I do not like the new operating system, I just want to avoid unnecessary hassle.
My older XP machines will not stand the strain of the new OS and my Vista box is finally working the way I want it to. Why change?
Windows 7 is a very nice OS, but upgrading and migrating is not as simple as it might be. Have fun shopping for a new Windows 7 machine.