Swiss Tony...Nick is right...region to region...there should be no 'extra' miles required for NW domestic upgrades. I recently did a trip to LAX via MSP and got the internals upgraded...(as part of the miles upgrade, not because of my PE status)
The round trip LHR/AMS/MSP/LAX and back cost me 40000 miles...in my book, a pretty good deal. (ps Shame that KL are handing over the AMS..MSP/DTW routes come April..means the usual hassle with getting upgraded seats with NW...on ancient DC10s')