rail travel

www.ratp.fr can give you schedules and a transit map of Paris.

Not to sound too much like someone name Sxxvx, CDG is on the RER B line and Gare St. Lazare is on the RER E line. You can transfer between them at Gare du Nord.

I've always used the RER to get into Paris, but last fall construction on the RER B line got me to try Roissybus (9,40 Euros). I'll probably never use the RER again (now going TO CDG, yes).

I don't know your age/physical condition, but if you're up for an approx. 4-5 city-block walk b/n the Opera Garnier (where the bus lets you out) and Saint-Lazare, it's a great alternative and you get to stretch your legs - in Paris! Take rue Auber from the back of Opera Garnier for a few blocks until you get to a major intersection - veer right on rue du Havre, and the station is right in front on you a block or so away (test this out on 'Google Street View' for fun!).

Or, you can take the metro b/n the two for two stops. The Gare Saint-Lazare metro street entrance is a glass dome 'poof' (!) at the far end of the front of Gare Saint-Lazare (you can't see it until you're at the intersection of rue Saint-Lazare and rue de Rome). The street entrance for the Opera Garnier metro stop is in front of the Opera.