Windows 7 launches this week and those waiting to upgrade should first decide if they want to install the Windows 7 64-bit (x64) version or stick with 32-bit (x86) Windows. Why? Even if you install 32-bit initially, it should be possible to upgrade to 64-bit later, right? Wrong.
You can’t upgrade from 32-bit to Windows 7 64-bit without doing a fresh “clean” install, which requires you to reinstall all applications from scratch. Therefore, you need to decide on either 32-bit or 64-bit before you install Windows 7.
While I recommend you go with 64-bit, read through this article to make an informed decision.