M.N.
You need to check your car manual to make sure you are able to use LATCH in the middle. Most cars do not allow this and if in an accident then its your fault if something happens to them not the car or car seat people.
How old are your kids? how tall? how much do they weigh?
Like another poster said, KEEP YOUR KIDS HARNESSED as long as possible. when in an accident, a 5point harness will distruibite the force all around the body. In a seat belt accident, the lap belt could cause internal bleeding. If your child is not sitting properly, your child will be a flying object. Seat belts are only good if our children sit in them right the whole time. They are many car seats out there in many budgets. Your best bet would be Car-seat.org. they they can tell you what seat would fit best for your budget, car, and child. I can not recomend a booster because my daughter is still harnessed at 4yr 6mo. but I have a list of carseats for you.
Graco Nautilus $170 when child out grows harness by height, will be used as high back booster. when child out grows hbb, then it will convert to no back booster. Harness is good for 6year, booster is good for 9years.
Britax Regant $280 harness only to 80lbs. britax is discontinu this seat because of the frontier. you may be able to get a deal on it.
Britax Frontier $280 When child out grows harness, use as high back booster. Also has side impact protection.
Evenflo Triumph Advanced LX $140 only harness to 50lb has been side impact tested
First Years Truefit $190 harness only to 65lbs. this is a big roomy seat. I played with one and fell in love with. My aunt got it for her grandson and they love it. Easy install
Radian 80 $250. harness only 80lb. This seat is very slim. in any car, you can get thress across. However the draw back is, it does not work in all cars. They is a new model coming out in January. You can get some great deals out on the old model befor the new ones come out.
Radian 65 $200 Harness only to 65lbs
I recomend you go to car-seat.org there are Car seat techs there along with other moms and dads. there, they can tell you what seat would work best in your car, and for your child. Please Please, for the safety of your children, put them back in a 5 point harness. I know my dr still says booster at 40lbs and forward face at 20lbs 1 year, but they are not updated with the lastes recomendations from AARP
(America Academy of Pediatrics) It is recomended to keep your child rear facing to the max limit of the seat. Also keep them harness longer.