what is your 2YO's height and weight? If your child is under 35 lbs, please consider the safety benefits of extended rear-facing
http://www.car-safety.org/rearface.html
There are now many great convertible seats (seats that can be used rear-facing to about 33-35 lbs depending on make/model, then forward facing to 40-80 lbs depending on make/model) that have high height and weight limits that will allow you to keep your child in a 5 pt harness until the child is old enough and mature enough for a booster - the Evenflo Triumph Advance (retail ~$129), First Years/Compass TrueFit (retail ~$169), Sunshine Kids Radian (retail ~$199 for 65-lb limit, ~$279 for 80 lb limit), and the Britax Marathon, Decathlon, and Boulevard.
The Cosco/Eddie Bauer 3-in-1 (e.g. Alpha Omega) seats and combo seats (harness to 40 lbs then can be used as booster) are not generally recommended by safety techs because the top harness *height* setting is lower than many other brands and most kids outgrow the harness by height before they're ready to use the seat as a booster.
If your child is already forward facing, the Graco Nautilus (retail $150) and Britax Frontier (retail ~$279) are excellent forward-facing only seats that can be used as a 5-pt harness seat up to 65 lbs (Graco) or 80 lbs (Britax) and/or about 19" torso height, then can be used as an excellent belt-positioning booster seat.
This link has a lot of good info and recommendations for seats in a variety of budget ranges:
http://boards.babycenter.com/n/pfx/forum.aspx?tsn=1&n...
hope that helps!