Oh good - the weekly OEM thread.
Let me get down to the basics for you.
OEM Windows is tied to the motherboard it was originally installed on.
There is just one "if" to this situation - if that motherboard fails and you have a replacement then the OS can be reinstalled.
There is no other situation in the EULA that allows for the moving of OEM software.
So, if in the future you decide to swap your motherboard you will need to buy a new copy of the OS.
The difference in price is £80 and £125.
It is really quite easy to see that if you plan on upgrading your motherboard only once during the lifetime of Windows 7, the retail version makes most sense.
At motherboard upgrade one your cost for Windows 7 becomes £160, comapred to the £125 you spent on the retail version.
If you have no intention of sticking to the EULA then the simple solution is not to buy Windows at all and to download yourself an illegal copy.
Moving an OEM license from one machine to another invalidates the license - so you would be no more legal than somebody who illegally downloaded.
If you've actually got any intention of buying Windows then buy OEM if you've no intention of upgrading the motherboard.
Buy retail if you've got the intention of replacing the motherboard.
Steal it if you plan on breaking the license agreement - no point paying for an illegal installation.