Giverny isn't on an RER line, it's on the SNCF Paris>Rouen>Le Havre train line. The stop is Vernon.Unless someone knows different, you'd have to get yourself from Versailles back into Paris to the Gare St-Lazare.

For Versailles, you get on RER C at the Musée d'Orsay, being sure that the train says Versailles Rive Gauche and not Pontoise, nd Versailles Rive Gauche is the last stop on the line.

You could try this public transport planning tool
http://www.ratp.fr/itineraires/en/ratp/recherche-avancee

Use the link above to figure out how to use public transport to get around Paris and to Versailles. Since parts of the www.ratp.fr website are in French only you can also use www.transilien.com or www.vianavigo.com. The website www.parisbytrain.com gives a good overview of how to use public transport in Paris.

There are some bus tour companies that will do Versailles and Giverny in the same day, I think maybe Cityrama and/or Parisvision and possibly others. You can google that information. There is a way to get to Giverny to Versailles that doesn't involve going back into Paris but you would have to sync two trains. You can take a train from Versailles-Chantiers to Mantes-la-Jolie. Then you'd have to wait here for the next train to Vernon. You can get info for the Versailles-Chantiers to Mantes-la-Jolie train on the website www.transilien.com. For the Mantes-la-Jolie to Vernon train you need to use the website www.voyages-sncf.com. This website is in French but should be easy to figure out. Whether or not you can sync up the trains at Mantes-la-Jolie is something you'll need to look into.

My opinion is that these are best done as separate day trips. It's possible to spend a whole day at Versailles if you're really interested in seeing the palace, gardens, grounds, Trianon and Hamlet. If you just want to tour the palace and spend a little time in the gardens it could be possible.

Look at the official websites for both places to learn more. If you want to visit these places buy your tickets ahead of time to avoid any potentially lengthy ticket lines.

Giverny:

http://www.fondation-monet.fr/fr/
http://giverny.org/giverny/giverny.htm
http://www.vernon-visite.org/rgb4/who_we_are.htm

Versailles:

http://en.chateauversailles.fr/homepage
http://www.versailles-tourisme.com/en/accueil.html

Thanks everyone.

I think we will end up doing these on 2 different days. Had no idea that Giverny wasn't reachable on the RER line.

yes, Maddie - essentially they are in different directions, so getting between them isn't easy.

and if you are going to Versailles after the Musee D'Orsay, you'll hardly have time to get to Giverney, let alone see it properly.