If she is at least 22 lbs, look into the "Britax Regent"! I LOVE THIS ONE!!! It is a forward facing "booster" that uses 5 point harness to 80 lbs. and 53 inches! This is what we bought our son when he turned 2. He had already out grown a traditional convertible style- coming in at 40 lbs, 40 inches. Most of the other seats we could find only used 5 point to about 45 lbs, and we new that we wouldn't be ready to place him in the shoulder buckle seats any time soon.
Pros: LONG life of usefulness- HIGH safety ratings- Comfortable (my son has sat in this seat for 3 hours straight, while awake, and didn't complain)- Affordable when you consider how long it will last (CA is moving toward 8 years/ 80 lbs in booster)- As opposed to many of the others recommended here, it uses 5 point harness all the way to 80 lbs, many of the others (including the Alpha Omega Elite) only use it to 40-45 lbs.
Cons: BIG! and bulky (If you do not have a bigger car, it may not fit)- Was on back order when we bought (due to such high demand)- A LOT of securing points to fasten (which depending on your view may be a pro)- Pricey up front ($269.99 at Bergstroms, where I also just read “now in stock”)
Hope you find what you're looking for, ~J.
BTW- it doesn't offer any recommendations, but has a lot of GREAT info- http://www.aap.org/family/carseatguide.htm