Advice on time between landing in Paris and getting train

And you'll be even more convinced of the right decision when you realise that you're talking about an expected time of arrival, and you're counting on a smooth procedure from the landed plane to the railway platform. I've had enough late arrivals and lost luggage in my life not to count on being able to get a train immediately after a scheduled landing.

FWIW, the RER + Metro ticket to Gare Montparnasse costs around 10€ each. A taxi would probably cost around 50€ (taxi rate into Paris from CDG may be negotiable).

The taxi from CDG is a flat rate of €55 to the Rive Gauche. If you decide to go to Gare Montparnasse, consider taking the Air France bus, which drops you off with less of a walk to the mainline train stations than the Metro does and also eliminates the need to make a connection. I would just stay in the airport unless you need to be in Bordeaux the hour or so earlier that the train from Paris will get you there, but I think that the odds are that, in the usual course of things, you would make the 3:21 train.

I would definitely consider the later, direct train to be a better bet. If all goes well, then you have more time to freshen up, get cash, have lunch, etc.

If you go from CDG to Montparnasse by rail and Metro, you won't see much of Paris, and going by coach puts you at greater risk of traffic delays. I would get the direct CDG-Bordeaux train even if it means hanging around at the airport.

Thanks for the advice! I booked the train from CDG!

I guess it's too late for my advice, although it's not really advice, and it comes with the caveat: "Don't try this at home." We arrived at CDG airport on March 31, the day of the general strike, and had a train to Bordeaux to catch at Montparnasse 3+ hours after our scheduled arrival time of 12:55 am. However, our train was cancelled because of the strike, and we could take any Paris-Bordeaux TGV that day. That relieved some of the stress. Then my luggage didn't arrive, so I lost 20-25 minutes there waiting and making a claim. When we finally left the terminal there was an Air France bus right there about to leave, so we got on, and for some reason it stopped first at Montparnasse rather than Gare de Lyon. All in all, the time from our plane's arrival to arriving at Montparnasse was 2 hours, and we were half an hour early for the next train (which was actually earlier than our cancelled train). Despite this, next time I probably won't leave only 3 hours between plane and train. Although we lost some time because of the baggage issue, we also gained a lot of time because it was a Schengen flight, hence no passport control, and it was a lucky break on the bus timing.