Do not buy an Alpha Omega. They are terrible booster seats and for the money you can find a car seat that will harness your child for much longer anyway. Its sold as the 'last car seat you will ever need' but its not.
Here are a list of car seats you can use for your little one. Remember rear facing is best, and the AAP, NHTSA and safekids recommend children remain rear facing until they reach the rear facing weight limits of their convertible car seat (35-45lbs depending on model, and until there is less than one inch of hard shell above the childs head.)
Britax Roundabout 50 ($160 at Target) will harness to 35lbs rear facing, 50lbs forward facing.
Sunshine Kids Radian ($200+) will rear face to 40 or 45lbs, then forward face to 65 or 80lbs.
Graco MyRide65 ($150) will rear face to 40lbs, forward to 65lbs. This is a great extended rear facing seat.
Evenflo Triumph Advance ($130) will rear face to 35lbs, forward to 50lbs.
Evenflo Symphony65 will rear face to 40lbs and forward to 65lbs.
First years TrueFit ($180-$200) will rear face to 35lbs and forward face to 65lbs. My 3yr old is currently rear facing in this seat at 36" tall and 28lbs. :)
Remember NO BULKY CLOTHING in the car seat! It will compress in a crash and leave yoru child basically unrestrained! Always take the coat off and buckle your child, then cover with a blanket or put the coat on backwards. I live in Minnesota and I have three kids, ages 8 (booster) 5.5 (harness) and 3.5 (rear facing harness) and NONE of htem wear jackets in teh car seats! Its takes an extra minute to take the coat off and put it back on when you get out ,but that extra minute could save your childs life in a crash. Every car seat manual will state that you cannot use a bulky clothing in the car seat.
For rear facing the harness slot should be just at or below the shoulders. The harness should be snug as a hug and the chest clip should be level with the armpits.
For forward facing the harenss should be just at or ABOVE the shoulders. Again harness snug as a hug and chest clip level with the armpits.
www.car-seat.org
www.safekids.org