Maryland Baby:Graco Nautilus. It eventually can be used with the seat belt too.
I second the Graco Nautilus. Great seat and it fits in small vehicles well.
Our 3 year old is off the charts for height, so we wanted to go with something that would fit her for the longest time. We got the Graco Argo. It is a 5 point harness carsteat, that converts to a high back booster and a backless booster. You can use the Harness up to 70 pounds or 52" (4'3") and you can use the booster set up to 100 pounds.
DD has outgrown every carseat by height well before by weight, so this was the best option for us.
We also have the Graco Nautilus 3 in 1 for our 3 year old. I just checked the product instructions because she is very close to 40 lbs and we still have her hooked up with LATCH. It said they can use LATCH til 48 lbs so we're good for a little while.
Anyhow, we love the seat.