http://answers.microsoft.com/en-us/windows/forum/windows_7-windows_install/is-a-windows-7-license-key-valid-for-both-32-bit/70d546cd-b6e3-44d8-a6c8-fd7feb7d1915
Is a Windows 7 license key valid for both 32-bit and 64-bit?
The keys aren't bit specific.
Yes, it is valid for either 32 or 64 bit, its up to you which architecture you want to use. Please note 1 key, 1 license. You can't install 32 bit one computer and install 64 bit on another computer using the same key.