I purchased a 'convertable' seat about 6 weeks ago when my son grew to 20 lbs. and am very pleased with it. (Acutally I purchased 3 so that we could install them in the grandparents cars as well!) I beleive it is called the Alpha Omega Elite made by Cosco. It was $159 at Target. (the nicest one Target had) I also saw it at Toys R Us for the same price. I think it goes up to 100 pounds so it should last for a while with him. It is rear facing until he turns 1 year.
Just remember to purchase a 'boost' (ours is a styrofoam tube about 3" thick). This is because, if your back seat in inclined at all, the seat is also inclined for forward facing. When it is rear facing, it needs a lift. You put this tube underneath and it will be level.
I noticed that someone recommended the Roundabout. That was rated very high in safety by Consumer Reports. The down side is that it won't grow with your child as the max weight is lower.