I'd go to Northrend as well. Money shouldn't be a problem as long as you don't waste it needlessly in the AH. You will NEED flying before you hit Icecrown and Storm Peaks. That being said, you should earn enough $ as you quest through the zones before you reach Storm Peaks to at least get the lowest level of flying and cold weather flying (1.7K total I think??) . Also, as Emesis said, you can't fly until 77 anyway.

If you feel the need to make some extra money, you could complete the "lower level" dailies that open up as you quest through Northrend. There are quite a few in each of the zones and the rep you gain is far more useful than what you would get in Outland! However, this may very well be less efficient than just questing.