E.C.
There's a lot of helpful info on the FAQ at http://boards.babycenter.com/n/pfx/forum.aspx?tsn=1&n... . The bulletin board host and many of the regulars are certified Child-passenger safety techs. There are now a lot more seats with high harness height/weight limits, much more than when my now-6.5YO twins outgrew their infant seats.
One thing to keep in mind is that the Roundabout will most likely be outgrown by *height* before your child reaches the 40 lb weight limit, so buying a seat with a higher weight limit will also give you a higher height limit. Other convertible seats besides the Marathon and its cousins (the Decathlon and Boulevard) that have high height/weight limits are the Sunshine Kids Radian65 and Radian80, the Evenflo Triumph Advance, and the Compass TrueFit. The Radians look less bulky because they're narrower and sit a bit lower on your vehicle's seat.
Up until this year I would've recommended a Marathon over a Roundabout because a Roundabout will almost always be outgrown before a child is ready for a booster seat (my twins outgrew their Roundabouts by height at age 4 but could still fit a Marathon now). But now that there are more high-weight harnessing "combination" seats (forward facing seats that can be used with harness to 65 lbs and then without the harness as a booster) like the Graco Nautilus, Cosco Apex65 and Britax Frontier, I'd say find a store where you can try out a bunch of seats to see which ones seem easiest to adjust and buckle and go from there