The only things that don't work on 64-bit Windows 7 are old 16-bit programs, old drivers (e.g. if you use a printer which doesn't have 64-bit drivers) and maybe a very few 32-bit applications that do something weird, though I've never come across one. There's also the issue mentioned above, where plugins may not exist for 64-bit IE if you use it. Otherwise there's no reason whatsoever to pick 32-bit Windows 7 over 64-bit if the CPU supports it. Unfortunately there are very few 64-bit Windows apps so far, but they'll become increasingly common and some games actually require 64-bit for best performance now because they want reliable access to more than 2GB of RAM.