OK, looks like there's RER C trains to VERSAILLES RIVE GAUCHE - CHATEAU every 15 minutes or so. It's the end of the C5 line. There's a million stations where we can pick up the train, so Jean suggests ST MICHEL NOTRE DAME because it's central (metro M4 St Michel). Let's try to meet on the platform around 11:00 for the 11:12 departure, and if we snooze a bit, there's another train at 11:28.
You need an Origine/Destination ticket to Versailles which costs 3.35 each way. That's Zone 4 so it's a Zone 1-4 ticket. I think you can buy 'em at any metro station and use the same ticket for your metro ride. (Anyone know for sure?) I noticed that if you find one of the touch-screen ticket machines and choose "buy new ticket", it gives you a button for Versailles-Chateau right on the first screen. Handy!
- ranjit