A.,
I just spoke with a car seat tech about this. I have family members with 2 or 3 year old children who are outgrowing either height or weight. At this age it's important that they are still in a forward facing 5 point harness car seat. The reason is that their bones are not as strong as you want them to be to switch them into a booster. (** side note: DO NOT use a 5-point booster, they're poorly made... and if you can't afford the higher limit 5point car seats the high-back seat-belt positioning booster is a MUCH better second option).
So I'd like to recommend 2 car seats to you that have GREAT ratings, and are tested at higher speeds than what the US recommends. The first is a Fisher Price Safe Voyage. It goes up to 55lbs forward facing! It has a think EPS fome and has steel reinforcement!
Next is the Britax Marathon, it goes up to 65lbs forward facing, has thick EPS foam and steel reinforcement. It also has all the bells and whistles like cup holder, storage compartment etc. This one is more expensive but has all the extras. Britax also makes another that goes up to 85lbs forward facing, but is again, more expensive.
Check out both of them at eopinions.com and it will give you all kids of info and link you back to either manfacutrer's site if your intrested in seeing them.
Good luck car seat shopping!!
C.