stephanie,
There's a lot to see in Barcelona and area so if your time and budget will allow, I'd plan for at least four nights. It's a bit difficult to recommend "must see" places, as everyone has different ideas on that. One definite sight I'd recommend is La Sagrada Familia and online reservations would be a really good idea. Also allow a few hours for wandering around La Rambla and be sure to have a look at La Boqueria Market.
If you plan on travelling with an iPad, Kindle or whatever, you could also download the E-book version of the Spain guidebook (current price $8.34 on Amazon).
One question though - are you planning to rent the car in France and drop in Spain? There's often a HUGE charge for dropping off in a different country.