You want your child to be AT LEAST 4 AND AT LEAST 40LBS to switch to a high back booster, and AT LEAST 6 to use a no back booster. However MN state law really sucks when it comes to carseat usage.... MN law states proper usage up to 4yrs/40lbs, and then there are no booster laws. I wish that would change, and I wish that the law was tougher on parents who don't use or misuse carseats.
That being said, the bare minimum you want to switch to a booster is 4yrs AND 40lbs. Obviously a 5point harness is safer (why do you think race car drivers use them!) so keeping her in the harness as long as possible is absolutely the safest option. She will outgrow the five point harness carseat when her shoulders go above the top harness slot, her ears are level with the seat back, or she goes over the weight limit of the seat.
If you want to keep her in a harness longer, here are your options on carseats that will fit her best for her age and size.
Cosco Apex65- ($130) available online and at Babies R Us. This seat will harenss to 65lbs, and then becomes a belted booster to 100lbs. Great seat for the money, but you must have adjustable headrests in your car to use this seat. Other than that quirk, its a great seat that will accomidate a larger older child in the five point harness.
Britax Regent- ($270) available online and at Babies R Us, USA Baby, etc. This seat will harness from 22-80lbs and accomidates most children up to age 8, and most alot longer. This is literally the 'king of all carseats'. Its large, comfy, and is like a big recliner in the car. WE own one of these for our 5 1/2 year old (he is 48'' tall and 45lbs). He has at least 3 years in this seat before he outgrows it.
Graco Nautilus- ($150) will be available at Wal Mart nationwide by the end of January. Thsi is Graco's new seat! It looks just like the Turbobooster only it has a five point harness that will harness to 65lbs, and then it becomes a belted booster to 100lbs, and will also become a no back booster. This is a GREAT seat for the money, and it will fit most children right up to age 6-8. This is the seat I would recommend to you if you are interested in extended harnessing, keeping your daughter in the 5point harness as long as possible.
Britax and Sunshine Kids Radian make seatss that harness to 65lbs, but at her age and size I would recommend you consider one of the above instead.
Again, at her age, a booster is ok, however a 5point harness IS safer, so the seats above would be perfect for your daughter.