Thanks again for all the tips so far.
My intent at this point, is to go right from CDG to Giverny then Rouen by train. (Must go to Paris and get on train at St. Lazare). Then return to Gare St Lazare, rent a car, drive to Beaune. It looks like there is a lot to see and do right there, plenty for two days. I'll save the mustard museum for another time. Seriously, I hope that there is another trip soon, and I can see the Loire valley and hike in Alsace. Of course, all this is subject to change, partly based on advice I get.

Speaking of change, that brings up a couple of questions specifically relating to the trains. You can buy tickets online here http://www.raileurope.com/index.html In the US, you generally walk up to a ticket booth and buy them there, or just get on the train and buy from the conductor (for a small surcharge). Other than not having to wait in line, what advantage is there to buying weeks in advance? Can you get shut out like a plane? On a related note, if you buy in advance and do not get the "flexible" ticket does that mean if you miss your train you are out of luck? If you get the flexible ticket and miss your train or want to change, do you go to a ticket booth, or just get on the next train?

Since we can't get an RER from CDG to St. Lazare, I will probably have to spring for a taxi. But can you transfer from the RER to the metro, or metro to the RER for the return trip?

I realize that the first class seats are more comfortable than second. A lot more?

Thanks again.