There seems to be no general agreement arround here on what should be called a naga or a lamia. Tags exist for both and Wikipedia says that neither of those words bear a meaning anywhere close to the creature we're trying to label (one with the torso of a human but the body of a snake).
I've seen them go by the word naga in more than a few books, but I've learned more recently that a lot of people call them lamias as well (such as the english fan translation of Monster Girl Quest).
I don't see anyone enforcing either term to designate men-snakes or anthropomorphic snakes.
If we could reach a consensus on what's what, that would make sorting much easier.

Fig A : (Wo)Man-snake
post #306924

Fig B : Anthro snake
post #306447

They are called lamias in FFXI.

Generally lamia is a half (wo)man/half snake, in centaur-ish style(sans legs for obvious reasons)

Naga can refer to the same, or more snake-like monsters.

Personally, I would say have lamia imply naga and call lamia a sub-species of nagas.

Also, you fail at wikipedia, http://en.wikipedia.org/wiki/Lamia describes a women with a snake's tail instead of legs as one of the forms of lamia, and http://en.wikipedia.org/wiki/N%C4%81ga http://en.wikipedia.org/wiki/Naga_(Dungeons_%26_Dragons) both desribe part-man/part-snake hybrids. As do the entries in L5R and WoW for naga.