I would recommend you look into a higher harness weight seat. A forward facing car seat is outgrown when one of three things happens, the child goes over the weight limit, the shoulders to over the top harness slot, or the tops of the ears become level with the top of hte seat back. When even one of these happens, its time for a new seat. Chances are, she's already outgrown it, as most seats are outgrown in heightl ong before weight, and if her shoulders are over the top harness slot ,she needs a new seat NOW.
The bare minimum for a booster seat is 4yrs AND 40lbs. Under 40lbs and the child is at a great risk of submarining under the seat belt, and at a much higher risk of seatbelt syndrome. Google both terms for more information. Its best to keep a child in a 5pt harness car seat until they are mature enough both mentally and physically to sit in an adult belted booster seat properly 100% of the time.
Here are a few seats that will harness her longer now, and then booster later.
Evenflo Generations 65- $99 will harness to 65lbs nad then becomes a high back booster. Try her out in this one, the top harness slots to see if she has enough growing room to make the purchase worthwhile.
Graco Nautilus $150 will harness to 65lbs and then becomes a high back booster, and eventuall ya no back booster, to 100lbs. This is a GREAt seat for the money, most kids can use the harness to age 6/7, and then it makes a good booster seat too.
Britax Frontier $300 will harness to 80lbs and then becomes a great high back booster seat. It will harness most kids to age 7.
If you insist on a booster seat for her now, make sure its a high back booster seat. Graco Turbobooster fits smaller children pretty well. We have a Sunshine Kids Monterey for our 8yr old, and its a great seat, very sturdy and amazing side impact protection.
When using a booster seat, make sure the child NEVER moves the shoulder belt, it should always rest across the chest and onto the shoulder. the lap belt should be low on the hips, if its up on the belly the child is not big enough for a booster seat. Children should remain in a high back booster for as long as possible, a no back booster provides ZERO side impact protection, and they need that to keep their spine aligned in a crash.
So stick wit ha 5pt harness as long as possible, but if you insist on a booster seat, please make it a high back booster, and make sure it fits her properly or it will do no good in a crash.