There is lots of great information about VERY safe car seats on this website: www.kyledavidmiller.org. (Go the the tab "Carseat Info" and "Our recommendations". It looks like one Cosco carseat made it onto her short list of very safe carseats.)
There you can read why it's safest to keep your child rear-facing for as long as possible, and THEN to use a 5-point harness for as long as possible (rather than a booster seat that uses the shoulder strap). The mother who started the website wanted to educate the public about carseat safety and raise money to buy safe carseats for families that can't afford them. Her three year old son was killed in a rollover accident, and she believes he could have survived if he had been in a 5-point harness. (He was properly secured in an age and weight appropriate carseat.)
You may be able to get a free carseat if you qualify as a "family in need". (Aren't we all?) Or you can buy a carseat off their store website (hipmonkey.com) and get 15% off plus free shipping on a variety of EXTREMELY SAFE carseats. (They have a short list, but they are top of the line for safety. Do NOT believe that person who said that all carseats are equally safe - you're just paying more for drink holders. Does anyone think a convertible sports car is just as safe as a minivan? MANY exceed the minimum safety requirements.)
I've got my 6-year old in a Sunshine Radian XT, which can use the 5-point harness until he's 80 pounds. It also has a steel frame - even the side-impact head wings are reinforced with 3 steel rods, which I haven't seen on any other models. It looks like you could drop this thing from a building and it would be intact. (No armrests or cup holders.) And yes, it was expensive, but I saved that 15% (no one else was discounting it because it's a brand new model), and I knew the money was going to help families in need.
One more tip: You also need to make sure the seat is a good fit with your car. www.carseatdata.org has some user ratings so you can see if your favorite carseat is compatible for your car. One of my cars has a VERY small backseat, so that was a big issue (hassle) for us.