I believe you need to be lvl 11 to enter shadow fang keep. That instance is made for boosting. Would go there asap.

I tend to stay a bit longer in the first tbc dungeon, till about lvl 63 after which I start with mana tombs.

In wotlk an awesome dungeon to boost is Drak'thraron keep, definitely if you have a tank. (since you can skip 2 bosses).