Consumer reports recently rated convertable car seats. I actually just purchased a Cosco Scenera NEXT car seat for my 18 month out. It was one of Consumer reports top 5 recommended seats and its designed to fit 3 acrosse, which I need with 3 kids. The only down side is that it only hold up to 40 pounds. However, as this is for my youngest, by the time he's 40 pounds he should fit in a high back booster.
Another option Cunsumer reports rated high was the Evenflo Sureride seat. It cost fron $85-100 depending on website. This seat works for kids up to 65 pounds. I seriously considered this one. However, my 6 yr old isn't even 65 pounds so I couldn't imagine actually using it that long. But for a first child it would have been great as I could have convinced them to be in a 5 point hardness longer.