Seats generally last 5-8 years...
Expiration depends on the manufacturer, but you should definitely NOT use a seat that is 12 years old. Standards have changed, so a newer seat would be safer, anyway.
A really nice seat is on sale at many Walmarts right now. It is the Fisher Price Safe Embrace (FPSE). It used to be $100, but now it's $60-ish. It has built in locking clips and EPS foam (like in bike helmets) lining the shell. Another seat to look at is the Century Accel (usually $60). Or, if money is no object, the Britax Roundabout is an *excellent* seat that runs $200. Toys R Us/Babies R Us is having a sale right now, and you could get the Accel or the Roundabout there for less than usual (they also have coupons for 15% and 20% off). You definitely want a seat with a 5 point harness (not a "shield" seat).
boards2.parentsplace.com/...7/6/3.html talks about these 3 seats. I really don't like the Touriva that is also recommended in that post. 1) It is *really* hard to use properly, and 2) the top harness slots aren't nearly as high as people first thought. They are actually much lower than the slots on the Roundabout, which means it can't be used as long as the other seats.
Also, you'll probably want to have the seat installed or checked by a technician. It's very easy to make a mistake, and instructions aren't always clear. You need to get the seat in TIGHT (and not all seats fit in all vehicles). You should be able to find a tech by going here.
www.nhtsa.com/people/inju.../Contacts/
I hope this helps you a little bit.