Your child should be at least 4yrs old AND 40lbs, and around 40" tall to be moved to a HIGH BACK BOOSTER SEAT. It is best to keep your child in a 5pt harness as long as possible though. If your child is under these bare minimums, keep her in a harness. If she's outgrown her current seat, find her a new higher harness car seat to replace hte one she is outgrowing.
A forward facing car seat is outgrown when one of three things happens; the child goes over the weight limit, the tops of the ears are level with the seat back, or the shoulders go over the top harness slot. If even one of these things happens she must be moved to another seat ASAP.
The Evenflo Maestro is $79 and will harness to 50lbs, and hten boosters. My almost 5yr old is 43" tall and still has room to grow in this seat, its a steal for the price.
Evenflo Generations 65 will harness to 65lbs, and then booster, and is $99.
Graco Nautilus is $150 and will harness to 65lbs and then booster to 100lbs. THis is a great seat that will last most kids to age 6/7 in the harness ,then booster!
If you insist on using a booster for her, make sure she is at least 4yrs old, 40lbs, and 40" tall, and that she is mature enough to not sleep in the car, not lean, stretch to the side or down and reach for things ,etc. That she can sit up straight and correctly the entire car ride, and not put the belt behind her back. Make sure its a high back booster seat, and that the belt lays across her shoulder and is low on her hips and not up on her belly. A proper fit is imperative. But in reality, a 5pt harness is safest.