Do not go to Northrend until you hit 70.
I wrote a post on this, but then I decided to just link you the source, since I wlll thereby do you two favors:
1) you will get a very clear explanation as to why it is a bad idea;
2) you will get a link to the very best leveling guides ever written for WoW, including pay guides.
Enjoy.
Jame's MMORPG Blog: Going to Northrend at level 68 - Is it a good idea?