15 posts and none of you even mentioned the biggest reason to buy Retail over OEM (or if so, I missed it!)
The OEM version is somewhat cheaper, but must be purchased as 32 bit or 64 bit.
The Retail version comes with both 32 and 64 bit media, and can be installed either way.
If you know which one you are going to run for sure and only plan to run it on a single system for the lifetime of the license, then buy OEM and save a few bucks. If you're not sure if your hardware will run 64 bit, hedge your bet by spending a few dollars more for the Retail version, so you can back off to 32 bit if necessary.
With respect to the licensing....
As has been mentioned, the OEM version is legally limited to the original computer system, and moving it to another system is a violation of the licensing terms you agreed to when you first installed it. Legally, installing it on another system even if it's no longer on the original is software piracy.
The Retail version is legally limited to being installed on a single computer system at any one time, so you can legally buy a new computer, and install the retail version on it instead of the original without violating the terms of the license agreement, and without committing software piracy.
And I do know someone who installed the OEM license on so many configurations that MS refused to re-activate, so it DOES happen. (What I don't know is whether or not they were using it on more than one machine concurrently, which is of course also piracy - but of a "more serious" sort.)
<S>