We just recently bought a larger car seat for our 6 month old (the infant car seats are usually only rated for kids up to 22 pounds). My online research into different models seemed to indicate that all commercial car seats must meet the federal safety requirements, but apart from that there aren't any rating systems for individual models. A few websites state that the "safest" car seat is simply the one that fits your child best, but that's pretty much useless information since you need to have the car seat before you have the child.
You can at least see which car seats have been recalled on this website:
Child Seat Recalls
If, for example, you notice that a particular mfr has a very high proportion of safety recalls, you would of course be wise to avoid that mfr.
(Our infant car seat was a Graco Metrolite; the new one is a Cosco 3-in-1. We weren't dealing with much of a space restriction--we have a Subaru Forester)