L.G.
Hi J..
I just went through months (literally) of researching what to do with the next seat for my 3 yr old, who was 40" tall and 38 pounds. Because of the things I learned and shared with the State Police when they were installing our seat for us (regarding our vehicles and our child), they asked me to become a technician.
You didn't mention how old, how tall or how much your daughter weighs.
The one thing I learned is that most car seat technicians recommend that you keep your child in a 5 point harness for as long as you can, and there are seats with 5 point harnesses that go up to 80 pounds. There are many sad and true stories about children who died using booster seats that were secured properly, but when their vehicle was in an accident, the seat belt failed. Kyle David Miller is one. http://www.youtube.com/watch?v=azgBhZfcqaQ
Many states are considering new laws to change weight minimums before a child is able to ride in a booster seat.
http://www.car-seat.org/
http://forum.carseatdata.org/
Both sites above are great forums moderated by trained and certified car seat technicians, and most of them are also parents. You can post, list your daughter's height, weight, seated height, the type and year of the vehicle(s) you own, whether or not you want your seat to portable from vehicle to vehicle, where in your vehicle you want the seat (by the doors or in the middle), how many other child/infant seats in the car, how many people (other than restrained children) will need to be seated in the vehicle on a usual basis, and those technicians will give you recommendations. They have thought of the things we don't even have an inkling about!
They can also give you information on best places to buy, when things go on sale (Britax sales are 2x a year, September and February, and they are significant sales!). Some of the sites will post where you can go to buy a seat on-line (avoid sales tax, etc), and return to a local store.
Long story short, check out those forums. I wish I had found them earlier in my search. Still, for me they confirmed my choices and helped me narrow from 4 to the two we purchased. I was really interested in the Frontier, but it wasn't coming out until June 08, and I needed the seat(s) back in February.
Good luck and safe driving/riding for you and your daughter!
Also! If you have financial difficulties and need assistance to properly restrain your daughter, you can apply for a seat through http://www.hipmonkey.com/
If you don't need financial assistance, and you buy your new seat through http://www.hipmonkey.com/, part of the proceeds goes to help provide safe seats to families in need. Talk about a win-win!