S.T.
I have had good luck with the Graco Nautilus. My six year old is still harnessed in it and has a little more room to grow. It goes up to 65 pounds harnessed (my six year old is pretty skinny - he's only 50 pounds) and then it can turn into a booster. He says it's very comfortable, and you can get it at Wal-Mart for around $150. I plan on getting another one in the next few months for my 3 year old since his carseat is about to expire. I'm just hoping I can fit both of them along with an infant seat into my second row of seats - we're due in July with our 3rd baby boy. Good luck!