The SNUZA Alarm: The Snuza Halo Monitor! WAY better than the Angelcare movement monitors because it goes WITH the baby. Only $100 for immediate peace of mind. It clips to baby's diaper and will alarm if it doesn't detect breathing for 30 seconds. It's loud enough, false alarms are rare, and will work no matter where baby is sleeping - swing, car seat, bouncy seat, when co-sleeping, etc.
Snuza Halo Baby Movement Monitor
Car Seat Safety:
From straps that are too loose to putting your child in the carseat wearing a winter coat, this blog entry is the best resource I have seen for car seat safety: