J.D.
I have a very tall girl as well and we ended up going with the Graco Natilus. It lets them stay in a 5 point harness up to 65 pounds, then converts to a backed booster, then a backless booster. I forget the height stats off the top of my head, but my girls comes up to over my hip (I'm 5'7) and she's only in the second from the bottom for her straps and has two higher positions we can move the straps up to.
It seems like this will be the last carseat we need to buy. We got ours from Amazon because we could get last years model(exactly the same a this year, just differant fabric) for less than anywhere in town and it qualified for free shipping.