If your primary goal is safety, then the studies show that the best thing to go is let her stay rear-facing (RF) for as long as possible. The sites below explain why in great detail (last 3 links), but what absolutely sold me was a mom's video (1st link):
http://youtube.com/watch?v=azgBhZfcqaQ
http://www.cpsafety.com/articles/StayRearFacing.aspx
http://www.car-seat.org/showthread.php?t=9316
http://www.car-safety.org/rearface.html
Many of the Britax seats have a RF weight limit of 35 lbs, and I'd recommend just that: buy a car seat w/ a 5-pt harness that will go to 35 lbs RF, and then to 65 lbs FF **w/ the 5-pt harness & tether**. I know that Britax (Boulevard, Decathalon) and Sunshine Kids (Radian) both have seats that convert from RF to FF w/ harness for kids up to 65 lbs FF. This is NOT about status -- I just haven't seen other carseats that allow you to use the 5-pt harness with a child over 40 lbs. The Sunshine Kids and Britax brands offer this ability in a convertible seat (one that can RF and FF). There are other brands that allow for harness over 40 lbs but these can go forward facing only (Safeguard, Recaro, Britax, Radian). I didn't buy my carseat because it was a Britax - I bought it because it was easy to install correctly in my car, easy to get my kids in and out, very easy to adjust to the right fit for my kids without having to disassemble the whole carseat, and was one of the only ones with great side-impact protection and a 5-pt harness & tether that held kids 40-65 lbs.
Also, don't let the prices scare you -- you will use this seat for MANY years to come. It cracks me up that people aren't willing to spend more than $50 or $100 on a carseat but those same people will think nothing of dropping that much or more on a cute holiday outfit or a winter coat that will barely last a year. A harnessed seat that goes up to 65 lbs will last for YEARS, and protect your child every time they're in the car.
My DD is 4.5 yrs old (42 lbs & tall for her age - people think she's 6 yrs old), and she's FF in a Britax Boulevard after outgrowing her Britax Advantage which only went to 40 lbs harnessed. The Boulevard will easily last us 2-3 more years (probably more), and it is SUCH a user-friendly seat (harness straps don't tangle, shoulder straps that are infinitely adjustable w/ knob, side-impact head protection, easy to route seatbelt or use LATCH). Plus, if you divide the price by the number of years you'll be using this seat, it's VERY reasonably priced considering the greatly increased safety a 5pt harness provides.
And yes - I'm one of those parents who will be letting my 6 or 7 yr old ride safely & comfortably in car seat w/ a 5-pt harness (not "stuffing them into" one, thank you very much!). :)