I have an Orange pre-paid phone as well and I use the 5 Euro cards to call home to the USA like you've mentioned. I'd imagine they have the same cards in France, but I don't know for sure. Still, if your pre-paid phone has a SIM from Spain in it, you're going to be "roaming" in France, which *might* mean you can't use the calling cards (or maybe there's some different rate)??? I'm not 100% sure since I've only ever used the cards from Spain.
I can tell you that I've always found the employees in the Orange stores here EXTREMELY helpful and very knowledgeable about the products they offer... I'm sure it's the same in France... If I were you, I'd probably just walk into a store once I got there and ask. I've tried calling the "Orange customer service hotline" before with questions, and have found they're not as helpful as the people in the actual stores, so I'd recommend checking in a store.
I hope that helps somewhat. Sorry I don't have more info for you!