You would be wise to do your research and homework (just like you're doing here) because if you don't you can wind up buying a new seat for every stage and go broke!
We switched our son to a convertible car seat at 9 months because he no longer 'fit' the snap-and-go infant carrier. His little legs were about to dangle off the end and the straps were getting very, very tight.
We wound up going with a Britax model (for the life of me, can't remember which but one of the middle models, not the top end or the bottom). I absolutely love it! You can use it rear-facing or forward-facing, it is totally easy to install, and the seat will accommodate our son until he needs to be in a booster.
Look at all the different offerings, see their age/height/weight range and requirements, then make a decision that best suits your family. Another good source is the book "Baby Bargains" because they evaluate pretty much every seat on the market.