I am in the same quest for my 16 month old. For my 4 year old, we have the Britax Regent and I really like it. The straps twist (which I was told never happens with Britax, but it does for many people now that I have researched more), but they don't twist as bad as other car seats I've had. My 4 year old loves his soft seat. It is wide and tall, so it will fit him hopefully until he is out of a car seat for good. I, personally, feel kids should be in a 5 pt. harness for as long as possible. It's safer.
Geri mentioned the available ones, but there are also some new ones coming out this summer/early fall. Recaro (they are right up there with Britax on safety- they make race car seats) is coming out with a Recaro Como and Signo that is a convertible that has a 70 lb. weight limit and 5 point harness until 70 lbs (Como should be out anytime now, Signo they are saying Aug/Sept). It'll be in the $200 price range somewhere. Evenflo is coming out with a Triumph Advance and Titan Advance that has a 5 point harness until 50 lbs. I have read some Babies R Us' have the Triumph Advance already in the store, and it is around $140.
I am personally waiting to see the new Recaro's. My 16 month old is too small for a Regent, but the other Britax's aren't tall enough for him (he's almost outgrown the height on those).
Another tidbit, Target.com (as of yesterday) is having free shipping on 2 or more baby items. Most of the car seats (check under the product details tabs to see for sure) you buy online can be returned to the store. So, they sell a lot of car seats on line that you can't find at almost any store around here, so I bought 2 and plan on returning one (or both) to the store. That way I can try them out without driving all of kingdom come to find a store that has it.
Good luck, what a pain- car seat shopping.