Hello Silentcom, welcome to the Seven Forums.
One retail or OEM pack of Windows 7 includes both 32-bit and 64-bit install media so you can decide which version you want to use. You can install and activate either of them but not both; only one working copy per key at any given time.
If you install and activate (win7 needs to be activated!) first let say the 32-bit version and later decide to replace it with 64-bit on the same computer, it's OK. Windows don't need to be "de-activated" but the when 32-bit is replaced with the 64-bit, the activation is kind of transferred to that later installed version. Same thing if you first install 32-bit to one computer and later 64-bit to another; when you activate the 64-bit, activation is transferred and the 32-bit is no longer activated.
So: One installation set includes both 32 and 64-bit versions and one key. You can install and activate one of those. If you want to use both, you need two keys.
Kari