Rick covers this in his Paris 2010 guide book, but I want to affirm how much easier, more accurate and much more pleasant these storefront offices of the French railway system are than trying to deal with the information and ticket booths at the stations. The stations are SO noisy and hectic, and the agents are often behind glass so they can't hear you very well and you certainly can't hear them, and not every agent speaks enough English to assist you. I was given completely inaccurate info at both Gare d'Austerlitz and Gare du Nord and was beginning to think my French railpass had been a huge waste of money.
Thankfully, a very pleasant and professional agent at a boutique solved all my ticket, routing and reservation problems in a quick 15 minutes today.
You may have to wait in a queue for a few minutes, but you get undivided, personalized and efficient service once your turn comes up -- in a quiet and businesslike atmosphere.
I think Rick lists addresses in the Paris guidebook, but here is also a link to Paris SNCF offices.