One of the things I have become pretty passionate about over the last three years is car seat safety. And, in fact, we are about to purchase a new car seat. We have considered moving Beanie into a convertible-to-booster such as the Britax Frontier or the Graco Nautilus & moving Bubby into her Britax Marathon. I'm not really sure she's ready for a new seat and the Marathon fit hers very well. In all likelihood, we will probably be buying a Graco MyRide for Bubby and leaving her in the Marathon. Yes, it's more money now. But, their safety is of the utmost importance.
All that mumbo jumbo to say Do your research before you buy a car seat. Don't buy the one you see at Costco because it's a good deal. Don't buy the one your friend uses because it works well for her kid. The infant bucket seat you buy (should you choose to use that instead of a convertible seat initially) will not last as long as you think it will. The convertible seat that says will last to 65lbs may not last because of your child's height. It is safest to keep your child rear-facing until they exceed the height & weight limits for their seat. It is best to keep them in a 5pt harness as long as possible.
Yesterday I saw a news report about new ratings on booster seats. Check it out.
Car booster seats make gains in insurance testing
Have your child test out the seat before you buy it. Test it out in your car. Have it checked by a certified technician after you install it.
Okay, stepping off my soapbox now ;)