this is a pretty good booster seat, i just bought it for my second car. ideally it is safest if you can keep your child in a 5 point harness for as long as possible. this seat keeps them in the harness till 47 pounds and then converts to a belt positioning booster till they are 100 pds or 57 inches.
http://www.evenflo.com/product.aspx?id=315&pfid=3452 ($130 approx)
or there is this one which i have in my main car. ($299 approx)
it keeps your child in the harness till 65 pounds and then a belt positioning booster till 120 pounds.
http://www.britax.ca/car-seats/frontier-xt
here is a pretty good thread on a car seat forum discussing highback vs backless boosters.
http://www.car-seat.org/showthread.php?p=236356