Whenever we recommend a build list for new systems, we inevitably turn to Windows OEM editions for the OS. They are bit for bit just as powerful as their retail cousins, but may require a bit of telephone tag with Microsoft when upgrading and you were stuck with nobody to call if you need support. Overall the experience wasn't so bad given the discount, but an important, albeit subtle change in the Windows 7 EULA could permanently alter this recommendation. The specific clause found in prior OEM editions of Windows is as follows:
"OEM system builder software packs are intended for PC and server manufacturers or assemblers ONLY. They are not intended for distribution to end users. Unless the end user is actually assembling his/her own PC, in which case, that end user is considered a system builder as well."
As you can see from the above passage, prior versions clearly made allowance for those that assembled their own system, sadly, this is no longer the case in Windows 7. Assuming this isn't a mistake (and when do lawyers ever make mistakes), then Windows 7 OEM editions can legally only be installed on machines you intend to sell. I suppose you could always pawn off your new machine to a family member for a song, then politely ask them to return it, but Microsoft clearly wants to push more home users over to the retail edition.
You can still buy OEM editions as easily as before from online retailers such as Newegg, but if your moral compass points true north, you'll need to buy retail editions on new systems you aren't selling from now on. Will this stop you from using OEM editions?