I got my team a bit of gear in outland before I started dungeons in northrend. My pally was partially raid geared, and was probably at the level to start Main Tanking Kara (Pre Nerf) so that definately made it easier. I am still wearing some of my epics tho, and I am 77... Weapons are easy northrend upgrades, but my chestpieces are still 70 epics. If I were you I'd fine some bored 80s and get them to run you through Kara once a week. Its a fun place, you get gear, badges, and gold.

But its up to you. I had a hard time in northrend dungeons, so I went back and did outland heroics to replace my quest rewards, and then I had a much better time.