I've been looking at car seats too, and it looks like the Britax Marathon is going to win the race. Yes, it is the most expensive one out there ($250), but it is for 5-65 lbs, is front and rear facing. I also looked at the Britax Roundabout, but the adjustments for the shoulder belts didn't go down as far as on the Marathon, so I don't think an infant would fit as well. All the comments on the sites I went to were positive for this car seat. It even has velcro on the side so you can stick the shoulder straps while placing baby in (they won't end up behind baby). If you are around a BabiesRUs, go have a look. I also know that Consumer Reports likes britax.
Hope this helps.