For belt positioning boosters (and all carseats this is the rule), the child is supposed to meet all of the requirements - age, height and weight - before it is used. What I've heard/read, the belt-positioning boosters really aren't supposed to be used until 4 years and 40 lbs, as a 5-pt is just so much safer. Some good seats that meet those requirements (we have the SafeGuard Go booster, though it doesn't have sides and is shaped more like a booster, and the Radian 65) are the Graco Nautilus, Sunshine Kids Radian, Britax Regent, Britax Frontier, SafeGuard Child Seat, and SafeGuard Go Booster. THey are all good in a 5-pt harness over 50". I'm sure there are others I'm forgetting, these are just the ones I've looked at and researched over and over. Elitecarseats.com is a good site for user and expert reviews on seats for pros and cons. Car-seat.org is also a good site for parent reviews.
I do particularly like the Radian, despite it being really heavy, because it's very tall, my daughter is comfortable, it's nice and narrow in the base (good when you have multiple seats), and it works very well on aircraft as well (though again, it's very heavy).