No - you can upgrade from anything to any version of Windows 7. 32 bit to 64 bit, Windows XP to Windows 7-Ultimate...whatever you want.
The OEM versions of MS software is intended for system builders or home builders like me (I buy the motherboard and components I want and build it myself). You get no phone support from MS and the operating system <span style="font-style: italic">cannot</span> be uninstalled and moved to another machine. In short: once you activate the OEM product on your machine it's there forever and can't be reused (although home-builders like me will tell you it can and has been done).
Since it's intended for computer builders only the "support" is supposed to be directly from the computer builder - not Microsoft. The amount of good quality support online doesn't, in my opinion, outweigh the extra money for a phone call to a support person that is probably in India. But if you're a person that calls an 800-number every time a message pops up on your screen then maybe OEM isn't for you.
OEM versions are also 32bit *OR* 64bit. You buy the one you want and you're stuck with it.
On the other hand the upgrade and retail versions can be installed then later if you computer dies. If you build/buy another one you can reinstall it and reactivate it there. Then there's that support from Microsoft I mentioned.
Upgrade and retail (except for the label on the box and the license I'm pretty sure they're identical by the way) can be installed as either 32bit or 64bit. They come with disks for either version. If you put 32bit on now and want to change to 64bit a year from now you can do it.
Laying it out here price-wise: Newegg (my benchmark for prices) has the Win7 Pro UPGRADE for $188.99. The OEM version is $139.99. For $50 you're getting the ability to reinstall later if you destroy your computer and build another, and you have access to their supposed "support".
For the "Home Premium" version (probably what fits the needs of most people) the prices are a lot different. The Upgrade version is $114.99 and the OEM is $106.99. For $8 I'd probably get the upgrade version but that's just me.
For people that want the "Home Premium" version and have more than one computer they're selling a 3-pack that comes with 3 full licenses so you can install it on three different machines. Newegg sells it for $149.99 and Sams Club has it for $147.99 - bringing the per-computer cost to around $50.
I don't know what the licensing limitation is on the 3-pack but if you've got three friends that want to upgrade maybe you can split the cost and upgrade for $50 each
