Deciding Between Windows 7 32-bit or 64-bit
Your organization decided it will go to Windows 7, and you're responsible for directing this project. One question you will have to face sooner or later is if you will install 32-bit or 64-bit version of Windows 7.
Pros
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>=4GB RAM support. If your organizaton's standard RAM issued on machines is at least 4GB, then considering 64-bit is a good investment for you to maximize your hardware -- that is if your CPU itself can support 64-bit. Consider all other factors though, especially the ones below.
Cons
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16-bit applications will not run. In fact, very old software will have issues running on very new OS, generally speaking.
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32-bit drivers will have problems on 64-bit OS. Drivers should be 64-bit
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Signed kernel mode drivers only, no less. Unsigned ones will not work
Bottomline, whatever works on 32-bit will not always work on 64-bit, therefore your Application Compatibility Testing exercise needs to be performed on both 32-bit and 64-bit version of Windows 7. To check out if your application or driver works for Windows 7 64-bit, check out the Windows 7 Compatibility Center, don't forget to click on 64-bit Windows 7 under Select a system type to check if the app/device also works for 64-bit.
DO Consider 64-bit in your next hardware refresh cycle only if these conditions are met
Don't even consider 64-bit now if you have standard RAM of 1-2GB even if CPU can support 64-bit. If you're upgrading to at least 4GB RAM in the near future, balance out the extra effort in appcompat testing 64-bit vs. reinstalling Windows 7 64-bit after the RAM upgrade. Deploying Windows 7 is a lot easier than its predecessors, especially with regards to User State Migration, with the Hardlink Migration feature -- but that's an entirely different discussion already.