This is an important question in view of the difficult economic situation we are currently facing. Here is a gereral rule of thumb. If the interview is informal and the setting is informal as well, then a woman does not need to dress up too much for an interview. On the other hand, if the interview and context is formal, then a woman should dress up.
The key point to this discussion is context. If you know the context, then you will dress appropriately. For a woman, a formal interview should entail a suit, shoes and accessories that match the attire.