Here's an interesting that happened to me with Windows 7.
I installed W7 Pro OEM 32 bit on my new rackmount server. My worry was that the software that ABSOLUTELY had to work would not run properly under a 64 bit environment, so I decided to purchase the 32 bit OEM version.
This week, a few things happened. I managed to scrounge enough information to discover that I COULD use the 64 bit version after all. But I didn't have the install disc.
Interesting information. All versions of W7 64 bit except the OEM include the 32 bit installation disc in case you need to fall back on it. The 32 bit W7 versions however don't include the 64 bit version, OEM or not.
But here's the clincher. The license key you purchase doesn't care if your W7 installation is 32 or 64 bits. All it cares about is that it matches the version you've bought (Home, Pro, Ultimate, Full, Upgrade, OEM...). This is also reflected in the pricing, where there's no difference between the 32 and 64 bit versions of the package you decide to install.
So I emailed Microsoft to try and get a 64 bit W7 Pro OEM alternate media DVD. They told me I could order it from the Canadian order desk, for which they gave me the phone number.
Surprise! Microsoft doesn't have alternate media discs of the OEM versions. They have it for everything else, but not the OEMs. I emailed back the rep. I previously contacted, but she couldn't authorize a shipment.
So I found and downloaded the disc's ISO from the Internet (original disc image only, no cracks included), burnt it, and installed the 64 bit version.
My original purchased license key was accepted and passed WGA.
In all my dealings with Microsoft, no one said that I couldn't legally install the 64 bit OEM version. In fact, the first email I received from the Microsoft rep. clearly stated that I could switch to the 64 bit version with no problems, and this after I clearly stated that I had an OEM version of the disc. All they couldn't do was ship me an alternate OEM media disc.
The disc image I downloaded is useless without a valid license key, which I already had, and which was to be used only on the same machine that had the 32 bit version. Even the license sticker only indicates Windows 7 Professional OEM with no distinction between 32 or 64 bits.
I told Microsoft I found another source for the alternate media disc, but never told them exactly where I got it. It'll be interesting to see how they respond to that, if they ever do.
So. Legal? Illegal? Gray area? From my point of view, I'm perfectly legit. I've paid for the license with my computer purchase, and I have the invoice to back it up along with all the emails.