Retail is licensed to you, to use on any single computer you want to, no restrictions. OEM is licensed to the hardware in the first system you install the OEM license on, it's not legally re-usable. If you're a mad platform-upgrade fiend and want to be legit, then obviously Retail is the better value in the long run.
Obviously you can install OEM and do a reasonable amount of upgrading to the system it's on. Where to draw the line on the system becoming a "new" system is simple if you're dealing with a Dell or HP or eMachine, but a bit hazy in the DIY market. The motherboard seems to be the tiebreaker item, although Microsoft should really be explicit about that.