You'll pay for a retail version, are you insane? The only reason to go with a retail version that I can think of is that you can completely legitimately transfer it to a new PC build without Microsoft asking tricky questions about board replacements.
Otherwise, OEM versions have no problems being activated on a different board, so I've heard
(if anyone asks me any questions on this topic, I shall deny everything!)
I could understand buying a retail licence a bit more for an OS that you know you're going to stick with for a long time, but for all you know, Win7 might not even work very well on your setup, or Win8 finally fixes the Vista legacy properly and outperforms WinXP on equal ground.
Even though the retail versions are extremely over-priced (compare with Apple OS licences for example), at least Microsoft don't seem to be engaging in the usual behaviour for creating UK prices by substituting the dollar sign for a pound sign.