I'm sure it depends on what type of carseats you're using. Most likely, to fit 6 carseats, you're going to need to get all new THIN carseats. I believe Radian is the company that makes really thin convertible carseats, but they're pricey.
If you're truly looking to get a new vehicle/new carseats, go to
www.car-seat.org. The ladies on there are wonderful with finding the perfect combination of seats to fit in any specific vehicle.