It depends... 1800's a good number to aim at, but depending on your spec and other stats you can do it with 1500 self-buffed (or even 1400, in fact). On the other hand, some people might manage to achieve 2000 spellpower but somehow lack the intellect for the place.
I've got 1650 before Inner Fire or any other buffs, and I'm fine in Naxx, both 10 and 25. Actually 10 is more challenging in a lot of ways; I tend to go through mana faster. My gear is, let's just say, a work in progress.