Hi M.,
When choosing a car seat for your children, you must make sure
1. Your child fits the seat
2. The seat fits the car, and,
3. Choose one that you will use properly every time your child is in the car.
Every time you "graduate" your child to the next category of seat, safety is reduced. So, a rear-facing seat is safer than a forward-facing seat, and a forward-facing seat (with harness) seat is safer than a booster. Finally, boosters are better than placing a too-small child into the adult seat-belt system (Google "5-step test").
Many new seats go up to 65# using the harness (most Britax, Graco Nautilus, and others), some go to 80# with the harness (Britax Frontier, Radian80). From a safety perspective, an 80# (harness) seat is the best choice. The Britax Frontier will convert also to a booster.
If you choose to replace the current seat with a new high-weight harness seat, the key measurement is your child's seated height -- from seat to shoulder. On a harness seat, the top harness slot MUST be AT OR ABOVE your child's shoulders. You will need to check with the manufactuers' site to get that top-slot information.
Whatever you choose, make sure you follow the manufacturer's directions (read the manual, Read The Manual, READ THE MANUAL!).
Finally, please have the seat's installation checked by a certified CPS Technician. You may go to the safekids.org web site to fine a seat-check event or individual technician. Most checkups are free.
There are many great resources on line ... here's just three:
www.carseat.org
www.car-seat.org
CPSP Yahoo Group ("Child Passenger Safety Professionals") at groups.yahoo.com
-- added note based on responses:
The child seat laws (such as 6 years or 60#) are MINIMUMS and do not take into consideration the size or safety of your child. Following the logic, if one insists on taking a child out of a car seat/booster at 6/60 or 8/80, then the child should be taken out of school at 16... after all, that's the law!
-- off soapbox
My best to all,
-- M. Cohen
Passionate Child Passenger Safety Technician/Instructor I-575114
Burbank CA
Loving Step-Father of M. Monique 1980-2002
Loving Grandfather of Alec Jaye 2000-2002
"Again this year, Alec and his mommy won't be going to grandma's for the holidays"