Daniel,
Since you've been to Paris before, you are the one who can best judge how many rides you're likely to take in a day. Here are some factors to consider. First of all, the Carte Orange is valid MONDAY-SUNDAY, not Sunday-Saturday. The current price for a zone 1-2 CO is €16.80. If you can't find a station selling the paper CO (it's being phased out), you'll have to buy a passe Navigo Decouverte (€5) on which you can load a CO. If you buy a carnet (€11.40), each ride will cost €1.14. Using €1.14 as the basis for comparison, a paper CO will pay for itself after 15 rides and a CO loaded on a passe Navigo Decouverte will pay for itself after 19 rides. A zone 1-2 Mobilis pass (€5.80) will pay for itself after six rides. However, unless you're going to take at least six rides a day on Friday, Saturday, and Sunday, buying carnets and sharing the tickets would be your best choice for those days. Three Mobilis passes (Friday, Saturday, Sunday) will cost less than a three-day zone 1-3 Paris Visite pass.
If you and your girlfriend are under 26, there is an even better deal for Saturday and Sunday. A zone 1-2 Ticket Jeunes costs only €3.20. It's a one-day pass good either Saturday or Sunday.
As Jona suggested, consider buying a Forfait Loisirs for Versailles.