okay, thanks for the advice so far.
horns, the site you linked ended up being "powered by" raileurope.com, which is the site i already posted.
i was told by a friend who lives there that the price will increase as the date draws closer, but i don't know by how much. in any case, i'll probably use raileurope.
if anyone cares there is another site that offers the same tickets for about $20 less, but it is in french, the customer service people do not speak english or spanish, and i see no provision for having tickets sent to the u.s. (the link on the first page does not work if you choose etats unis.)
http://www.voyages-sncf.com/billet-train