The Train à Grande Vitesse is a comfortable and low-hassle way to travel around the country, and reservations can be made online from the States with no fuss. However, you should know the following two items.
When reserving online for the TGV that departs directly from Charles de Gaulle airport (Paris), the names "Roissy (95)" and "Aeroport CDG 2 TGV (95)" both refer to this station. This was slightly confusing to me; I just made my reservation for Roissy and crossed my fingers. Incidentally the station looks like this:
In order to pick up your ticket at the station, you will need the credit card that you originally used to buy your ticket. You may be tempted by machines that look like this:
...which seem great, except for the following minor glitch, if you're a U.S. traveler:
As most U.S. credit cards are currently magnetic stripe-based, you will have to wait on line at the ticket counter. The ticket counter is slow, so slow that the French guy on line in front of me remarked "Le train, c'est très vite, mais ça? Pffft."