Personal experiences trying to wedge two carseats into the back of my car:
1) You really, really want something that is truly 4-door. Wrangling an infant carseat into DH's old 2-door Mercury Cougar was a royal pain in the neck, and even dealing with my clamshell 4-door Saturn Ion is less than ideal. The tradeoff is I can park next to someone and get the kids in and out because my back door is so short - DH has more trouble in his Hyundai Sonata because the doors are longer (but easier to get kids in and out of when nobody's parked next to him).
2) Anything with a bench back seat is going to be useless for everything except car seats, even if there is theoretically a middle seat back there. Car seats take up more horizontal room than most people do and they're a pain in the neck to climb over (even just the car seat base for infant seats), which means you can maybe squeeze a very skinny person in between them for a < 5 minute ride but they probably wouldn't like you much afterward.
3) You probably transport extra people more often than you think you do. DH and I have only been doing the two carseat thing for around seven months now, and we've already had a dozen times where we wanted to pick up someone for an activity (because of course we always have to drive when carpooling, since we're the ones with the carseats) and we end up having to take two cars because we can't physically fit more than two adults plus the two carseats in either of our cars despite technically seating five.
4) Bucket seats are a pain in the patootie to get car seats into. Even when you do get the carseat in properly, you have to stick a cut-up foam pool noodle underneath it to make it settle correctly and then you have to adjust it all the time because the angle gets off after a month or two. If I had known how much trouble it would have been, I would have gotten a car with a bench back seat without the buckets.
5) Whatever size your trunk is, make sure it's the right *shape* to fit a folded-up stroller (or two?) plus a load of groceries. Being able to fit a stroller plus a regular-sized suitcase is a plus.
6) Should be obvious, but it's worth stating: if you're getting a bigger car than you have now, make sure it fits in your garage before you buy it