trains from Paris to Giverny & Versailles, and parispass

Paris pass is almost always not worth it. Get a 4 day Museum pass, buy it there ( no need to prebuy and pay for shipping etc) . The ticket to Versailles on the RER is 7 euros return ( I would not do both Giverny and Versailles in one day but it can be done apparently) . Ticket to Vernon ( Giverny) is around 14 euros( not sure). The four day Museum pass is not a bad deal if you visit at least 2 sights a day, and it can be used for Versailles( and thats a minimum of 18 euros there) .
When is trip, Monets Gardens close Nov 1st

I'm not sure it's physically impossible to do Giverny and Paris in one day, but if not, it's pretty close. We did Paris-Giverny and back in a day trip with ease, but I don't know how we could have gotten to Versailles also, unless maybe we started at 5 a.m. Paris-Giverny took a change of trains and then a taxi from the station to the garden.

I agree that the Paris Pass is not a good deal. You can do much better buying a Paris Museum Pass and paying for transportation separately. I wouldn't want to squeeze both Giverny and Versailles into one day, but here's how you could make a whirlwind visit to both places. Take the RER C from central Paris to Versailles-Rive Gauche in time to be at the chateau when it opens at 09:00. Be aware that the RER C line divides on the west side of Paris so be sure to get on a train that is going to Versailles-Rive Gauche. It will have the nameplate VICK. Trains from Paris to Vernon (the station closest to Giverny) depart from Gare Saint-Lazare. You'll need to leave Versailles-Rive Gauche by 13:00 in order to get to Saint-Lazare a little before 14:00. Take the RER C from Versailles-Rive Gauche to Invalides. Transfer to Metro line 13 and take it to Saint-Lazare. Leave Gare Saint-Lazare at 14:20 and arrive at Vernon at 15:04. Take either a bus or taxi from Vernon to Giverny. Return to Vernon in time to catch the 18:53 train to Paris. For a zoomable map of the Metro and RER system, go here: http://tinyurl.com/6xmvms4.

Here is site for trip planner to work out schedule for hitting your sites. http://www.transilien.com/web/site/accueil/etat-trafic/chercher-itineraire/lang/en Depending on your timing, a Navigo Decouverte Semaine pass would be a great deal. Just did this 2 weeks ago and it worked slick. Cost is 33.90 Euro for one week (Monday to Sunday) zones 1-5, plus 5 Euro initial processing. Need to bring a 1" by 1.2" photo (just take a digital photo of yourself and reduce it to apropriate size and print on photo paper). We got these at the SNCF ticket center at CDG TGV station. Looked like a big line, but we got through it in 15 minutes and it only took 3 minutes to get the pass issued. Now we have a pass good for 10 years and we just reload it next time we go to Paris. Zones 1-2 are only 19.15, but with train from CDG to Paris and trip to Versailles make Zone 1-5 worthwhile.
Here is the site, only available in French, so use Google Translate to make sense of it. http://www.ratp.fr/en/ratp/c_21137/forfaits-navigo-mois-et-semaine/