You can easily get to Bordeaux on the day you arrive at CDG and you can nap on the train.
Direct TGVs from CDG to Bordeaux depart at 09:12, 13:37, and 16:37. Note, however, that the 16:37 train doesn't run on Sundays.
Yes, it will take about two hours from the time your plane lands to the time you will get to Gare Montparnasse: up to one hour to retrieve your luggage and go through immigration/passport control and a little under an hour to take the RER B to Denfert-Rochereau and metro line 4 from there to Montparnasse-Bienvenue. The fare is 8.70 EUR.
What you cannot control, of course, is your flight's arrival time. If you are willing to pay full fare, then just go into Paris, buy your tickets, and take the next available train. If you want to get a non-refundable, non-exchangeable Prem's fare as low as 25 EUR, then you should allow a minimum connection time of three hours (four would be much better). If your flight is late and you miss your train, your tickets will be worthless. Prem's fares are also available on the direct CDG-Bordeaux TGVs. Book ASAP (up to three months allowed) at www.tgv-europe.com. To keep the site in English and to avoid being bumped to the Rail Europe site which doesn't offer discount fares, choose Great Britain as your country of residence.