What comes after RA for toddlers??

I will be moving my 16 lb. 2 month old (yikes he's HUGE!) to my daughters RA soon, and would like to get her a new carseat first. She just turned 3 and weighs 31 lbs and is VERY tall for her age. What type of seat should I sart looking at? I was looking at a Summit today and was wondering if I'm heading in the right direction? I want to take her to the store and let here pick it out, but would like some info before we go so I can give her a choice of two or three. She's EXTREMELY picky and not willing to share any of her baby stuff with her new brother, so I need to do this soon to get her used to her new seat before my son reaches the 20 lb limit.

[Joolsplus2]

The Summit has rather low top slots, same as your RA, so might not be a good choice for a tall kiddo. The Graco Ultra or Platinum Cargo and the Evenflo Chase have top harness slots around 17 inches tall, versus the Summit's 15 or so.
These seats will harness till 40 pounds, then become fine boosters, or get handed down to your ds when HE'S tall but under 40 pounds.

Other choices would be the Britax Marathon/Wizard or Husky, which harness to 65 and 80 pounds, and have top slots at about 18 inches and 20 inches, respectively. They are pricey, but since you can hand them down, and may well have a much heavier little boy on your hands (turning 40 pounds before age 4 and having to go into a booster seat before he's mature enough for one is the worst-case scenario), one of these might be a good and cost-effective seat in the long run.

Either of those choices would be fine for your dd for now, though (Britax or Graco or Evenflo), it just depends on which you and she love best and fits your car and your budget.

:)
Julie CPS Tech and mom to 2 in seats
http://www.cpsafety.com/articles/RFAlbum/SarahMA.aspx