It all depends. OEM doesn't require a previous licensed copy of Windows. You get a license with the OEM. However, you get NO support from Microsoft. Also, you only get the 32-bit or 64-bit copy of Windows 7, so you have to decide at the time of purchase which one you want.
Also, once you activate the OEM copy, it is tied directly to your motherboard. Therefore, as per the license, you are NOT allowed to transfer it to another computer, even if you replace your motherboard. If you attempt to install it on another computer or replace your motherboard, you will have to activate the copy. However, it will error out saying that the copy of Windows is already activated on another computer and cannot be activated on this computer. You can attempt to call into Microsoft and see if they'll reactivate it for you, but that is a violation of the license agreement.
With the retail upgrade, you get BOTH 32-bit and 64-bit copies of Windows. The key is NOT tied to just one installation like the OEM version is. In other words, you can install it on one computer, activate it and then if you have to change the motherboard or you get a completely new system, you can install it and activate it. It will say that the computer configuration has changed so you'll need to call into Microsoft and get a new activation code. It's automated and takes about 5-10 minutes, but you generally never have to talk to someone to get it activated. This is NOT a violation of the license at all. You are allowed to switch it to another computer. You just can't run it on two different computers. This includes installing the 32-bit version on one and the 64-bit version on another computer. The same key is used for the 32-bit and 64-bit version. You just have to decide which one you're going to use.
Now, the kicker with the upgrade version is, you MUST have a previous copy of Windows Vista or XP that has a valid, activated key. Also, Windows 7 upgrade "requires" that you already have the previous copy installed and activated on the computer you wish to install the Windows 7 upgrade on. However, you can get around this and do a clean install of the Upgrade version. Some feel this a violation of the license agreement, especially because it allows someone to purchase the Upgrade copy and install it, even if they DON'T have a previous copy of Windows. Just search for how to do a clean install of Windows 7 upgrade.
Hopefully, that helps!