I realized recently that I was in the same situation with my daughter. After doing some research, I determined that a high-back booster with a 5-pt harness is the safest way for a small child to ride in a vehicle. I also felt very weird about having her fastened in with only a seat belt (and booster seat, of course). My research found three booster seats that use 5-pt harness for children over 40 lbs.:
Britax Regent (up to 80#), approx $249
Graco Nautilus (up to 65#), approx $149
Safety First Apex (up to 65#), approx $129
(if anyone knows of another such seat, please let us know, as I need to buy one more for the other car.)
I found a Britax Regent (in the color I wanted - yay!) on Ebay for about $200. Because I will be able to get so much use from it (my daughter is only about 36 lbs now), I was able to easily justify the cost to myself - not to mention the fact that she is much safer. She happens to LOVE the new seat, too.
FYI, I found a few short videos on youtube during my research that, while they are a little depressing, make very good arguments for using 5-point harnesses over regular booster seats with the vehicle's lap and shoulder belts. If you're interested in seeing any of them, search on youtube for 5-pt harness or booster seat.
Good luck on your search!