The first rule would be not to use the RailEurope website until you have checked the official French rail site www.voyages-sncf.com. It's English language companion site is www.tgv-europe.com. The RailEurope site usually has higher prices and doesn't show all available trains.
If you decide on this you'll need to use the TGV website to buy/reserve your tickets. For info on these trains use the website www.tgv-europe.com for schedules and prices (it's the English language version of www.voyages-sncf.com). Don't forget to uncheck the box marked "Direct Trains" to see all possible options. If the TGV site redirects you to the RailEurope website then try again and enter "Antarctic" as your ticket collection country. Other countries may also work but just try to find a country that doesn't get you redirected to RailEurope. The RailEurope website often doesn't show all the trains and generally has higher prices but you can check just to compare. For trains that require reservations you can buy tickets up to 3 months or more in advance and the earlier you buy them the cheaper they will be. The discounted tickets are known as PREM tickets but these are non-refundable and can't be exchanged.
Here is a website that will tell you how to navigate the TGV website:
An excellent website to learn all about train travel in Europe is www.seat61.com.
Unfortunately I can't log onto the SNCF and TGV websites right now to check your route as they always seem to be down late at night (my time) for updating. I have a feeling your only option to get to Tours from Caen is through Paris. Trying to take a train from Caen to another destination in Normandy that has a direct connection to Tours will likely take you longer than just going into Paris.
Edited: 7:18 pm, November 15, 2012