I have 3 kids, here's my thoughts:
Baby/infant seat: Buy the graco stroller travel system.
http://www.amazon.com/Graco-Passage-Travel-System-Morgan/dp/B002RL7US0
With it you will get:
-Baby seat
-Stroller that the baby seat clips into, that will eventually be able to hold a larger child
-Seat base
THEN, add to this purchase a spare base for each additional car. $30.00 or so:
http://www.amazon.com/Graco-SnugRid...s=baby-products&ie=UTF8&qid=1328287456&sr=1-1
This baby seat will be rear facing only
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Child/toddler seat: Graco Comfortsport convertible infant/toddler seat. cost is around $100
http://www.amazon.com/Graco-Comfort...s=baby-products&ie=UTF8&qid=1328287522&sr=1-3
We've had two of these, and the quality/ratings are high. These are seats that will rearface, and then be able to forward-face when you're/they're ready.
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Toddler/preschooler seat: Cosco 5-point booster. Around $40.00 @ walmart/target
http://www.amazon.com/Cosco-Juvenil...s=baby-products&ie=UTF8&qid=1328287603&sr=1-6
This seat has high ratings and is as big as it needs to be and as small as possible.
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Our oldest is six, so we've had a long time to form our opinions on these products, and their cost vs. value. I know some will put thousands into their child restraint systems but honestly the ones above have been beat to crap in our cars and by our kids, and are still very functional. By the way, that graco stroller is the one we choose when we go to theme parks even though we have a graco double (front/back like an f14 fighter) stroller. It will hold 3 kids with creative placement which is especially useful at the end of the day when everyone is pooped out.