Okay, I know you said you were on a budget, but......my friend has a convertable Recaro that you can only get online. It is absolutely the best carseat I have ever seen in person and I will be getting one shortly for my 11 month old. It is designed by a company that deals in racecar safety. The material is awesome; it looks like it would be sweaty but it's not. It is so soft and comfortable. The convertable grows, if I remember correctly off the top of my head, to almost five foot and 90-120 lbs forward facing, and from 20 to 35 or 40 rear facing (for comparison, my 3.5 year old is considered a 'big kid' and he is 33 lbs). My friend's DD is almost 7 (a fairly tall and 'big boned' child) and she has yet to have to turn it into the backless booster.
This said, it is fairly expensive up front, but for ten years of comfortable use, for our family, the 269.00 plus the potential to hand down is worth it.
Recaro has it's own website, but right now there is a deal on the website (www.babyviva.com) that has 15% off and free shipping!