My recommendation for OEM copies is to save money for family and friends, or customers that may want you to build a computer for them. They are not going to swap out a motherboard, and they are not going to call MS if they have a problem, they are going to call you, because hey, you are the family nerd, that's what you do.

For our systems, seriously, get a retail copy. If your an enthusiast you won't regret it. I don't have a crystal ball, but I'm going to guess that Windows 7 is going to have some staying power, its going to be around at least four or five years, and no doubt in my mind that any real enthusiast is going to want to do at least one motherboard swap in that time, and very possibly two or three if your a real hardware nut.

OEM for friends and family that just buy a new computer every five years, retail for the enthusiast.