If NW was going to replace their 747-200 fleet completely, buying the 747-400 series may not be such a bad idea, especially considering that flights to Asia tend to be full most of the year.
But I think NW should consider buying 20-25 747-400ER's so they could fly MSP-HKG or DTW-HKG non-stop with no payload restrictions on a year-round basis. That way, NW won't have to pay a premium for the 747X series, nor will they have to pay through the nose for the A380 series.