Radians are in the same price range as Britax seats.
The stated height limit is not always what the manufacturer actually goes by. Dorel seats do usually have a strict 40" height limit for rear facing (RF). Most other seats say that you can keep your child rear facing as long as the top of their head is at least 1" away from the top of the seat (provided they are also under the weight limit). For forward facing (FF), a seat is outgrown by height when the child's shoulders go above the top set of harness slots.
So, to tell how much height growing room you have RF, you usually (except for Dorel seats - Safety 1st, Eddie Bauer, Cosco, etc) want to compare how much space is above the child's head. To tell how long a seat will last for height FF, you want to see how tall the top set of harness slots is. (You can actually bring a measuring tape into the store with you to compare, if you want.)
Bottom line is that the stated height limit doesn't always tell the whole story.
As for specific seats... since you asked about the Peg Perego, I'll start there. The Peg Perego doesn't seem to give much more growing room for height than the Britax convertibles (when RF), from what I've seen here. I think they are also comparable as far as FF height room.
The Radians that the pp mentioned do tend to give more growing room when RF, possibly some more when FF as well. However, they do take lots of front-back space when RF unless you buy the angle adjuster, sold separately (about $15). They also can be tricky to install, impossible in some cars, but maybe you'll get lucky - they are easy to install in some cars. (What year/make/model of car do you drive?)
The Truefit is well-liked by some parents of tall/skinny kids. However some skinny-shouldered kids don't fit all that well in the harness.
There are a few tall Dorel seats that might be worth a look - keeping in mind their 40" height limit for rear facing - the Maxi Cosi Pria and the Safety 1st Complete Air.
For Graco, look at the Size4Me/MySize 70 and the MyRide. They are nice seats; the main complaint that people have with Graco these days is that the buckle that they use (which is a relatively recent change) is giving some people problems.
ETA: Occasionally, a salesperson at Babies R Us really knows what they are talking about with car seats, but usually they don't. So honestly, I wouldn't really ask them for advice, or take advice that they give with anything less than a few pinches of salt!
