I will be arriving at CDG on Sunday, Oct. 20, 2013 at 7:30 AM with rail departure from Gare de Lyon 3 hours later. I have read that it is best to take RER B from CDG, but is it best to transfer at Gare du Nord or at Chatelet-Les Halles in order to get to Gare de Lyon? I will have only carry-on luggage - so won't have to wait for my luggage at CDG. Where is it best to transfer (Gare du Nord or Chatelet) and is 3 hours sufficient time on a Sunday morning? Thanks.
Easiest is to get off at Les Halles and change to the RER, either A or D, that way you have 2 choices of RER lines to proceed. Just check the overhead monitors and go to the track showing the next departure toward Gare de Lyon. End points should be one of these: Marne-la-Vallee-Chessy, Boissy-St Leger, Melun, Corbell-Essonnes or Malesherbes. Changing at Gare du Nord your only choice is the D line.
In the final analysis, we're talking 5 to 10 minutes difference getting to Gare de Lyon. 3 hours should not be a problem. If you can get the tickets out of a machine, that will definitely save time as the wait for a ticket agent can be a half hour or more. All the machines take euro coins, but only some take bills. All take credit cards, but must be Pin&Chip type. Andrews Federal Credit Union is the only US issuer of this type of credit card. The fare is 9.50 euro. http://www.ratp.fr/fr/ratp/r_29246/dans-le-reseau-ratp/
There is a bus from CdG non-stop to the front of Gare de Lyon. However, "non-stop" during morning rush hour is not exactly accurate since Parisien traffic can be stop-and-go along the entire route. Even so, if you have your tickets in hand you can find your train quickly and there are no formalities; just get on board (remembering to validate your ticket in a machine if it's not electronic.) The bus is Line 4 on Les cars Air France http://www.aeroportsdeparis.fr/ADP/en-GB/Passagers/Access-maps-car-parks/Paris-CDG/Access/public-transport/paris-cdg-car-air-france.htm This site estimates the time at one hour, 15 minutes but I took closer to 1 hr. 45 minutes.