We are going to Spain in Dec. We're flying from NY to barcelona, spending three days, and then flying on spanair to seville for 2 nights. Then we need to get to spain where we'll be for three days and fly home from there. I know it's a lot but it's our first time in Spain. My question is what is the best way to get from sevilla to madrid? the Ave looks fast but is pretty expensive. Is it crazy to rent a car and drive, or should we fly? thanks

I mean we need to get from sevilla to madrid

That trip sounds really stressful and way too rushed. For a first time to Spain one should really focus on either Barcelona/Catalunya or Madrid and Andalucia. For a first time to Spain, Madrid, Seville and Granada is almost guaranteed to be a fantastic trip. For me Barcelona is just okay. I know you weren't asking about this so, If you're intent on that trip, the AVE is by far the best way to travel anywhere in Spain. I would not rent a car from Seville to Madrid.

If you are in Spain between the 4th and 6th of Dec you will be here during a National Public holiday. All means of transport will be busy, some hotels could be full and some museums will be on Sunday opening times at the best. Or are you here over the Christmas period?
You say you will be in BCN for 3 days, but one of those days could be lost to jetlag and if later arrival little sightseeing time. They you will loose almost a day going from BCN to SAV.
I would suggest you look at stay in BCN (as that is where you fly into) as there is more than enough to see in your short time here in Spain.

visitingspaindec writes re. Sevilla-Madrid: Is it crazy to rent a car and drive, or should we fly? thanks

Yes! Renting a car is not a good idea; and a car can be a burden in a City like Madrid. Monitor www.renfe.es for specials (AVE fares); hopefully you'll find something less expensive than their regular published fares. Unfortunately, most of the 'fare discounts' are for roundtrips only.

AVE runs hourly between the two cities; it's not a 'once-a-day' thing, so even if you do have to pay regular fare, it takes you from Sevilla Santa Justa to Madrid's Puerta de Atocha. Atocha is much more centrally located than Barajas airport, so you should save some on a taxi to your Hotel or Hostal.

I neglected to say that AVE Sevilla-Madrid takes 2.5 hours. A somewhat slower alternative would be Motorcoach (Bus) that does that route in 5.5 hours; departs Sevilla Plaza de Armas and it takes you to Estación Sur de Autobus, one Metro stop south of Atocha RENFE.

I've done that route on AVE, on Bus (Socibus, SA, & Secorbus SL). Bus is a lot cheaper; I would recommend an overnight Bus ride because there's not much Scenery along the route