Great suggestions from other mamas! We've done tons of research on carseats for our kids over the years . . .
We used the Britax Marathon with our son, but warning - if you have a tall child, or even average height, he won't be in it until 65 lbs! Britax are great seats, though pricey.
We have the Graco Nautilus for our 5.5 year old, he is still harnassed, which is important to us. But the Graco Nautilus is NOT a rear-facing seat,. I bet our son will fit in the harnass for at least another year, and it does convert to a high-back or low-back booster. As someone else mentioned, I wouldn't worry too much about the booster part. Boosters alone are SO cheap. Instead, think about how long you want your child in the 5-pt. harnass (the gov't is recommending it be longer than 4 years, though laws have not been changed). You can get a seat that has the 65 lb. harnass, but because of height, your kid will outgrow it long before.
Our 3-year old daughter is very tall, and just outgrew her regular convertible seat, we bought the Evenflo Syphony 65 for her, which harnasses until 65 lbs. and has an AWESEOME system for raising and lowering the shoulder harnass, you don't have to take the seat out and rethread. My 45 pound 5 year old also fits in this one (though not for long-this seat is not grow as "tall" as the Nautilus), and when baby #3 comes next spring, we'll be able to use it rear-facing as well. The latch system on this model is SUPER easy. The cup holder is cheap and lame, but oh, well. This seat has a 7 or 8 year suggested life instead of the usual 5.
Yes, alot of these seats are pricey, but totally worth it!