I definitely think you are making the right decision on getting new seats. As far as your 10 month old goes, I wouldn't waste your money on the Roundabout. She won't last in it very long. I would get either the Britax Marathon or Boulevard, or the Recaro Como or Signo (Signo isn't out yet, but the Como is and I have seen pics of 3 months old and older in it and it looks awesome). These seats will last until she is at least 3 years old or longer. If you want a little cheaper, the Fisher Price Safe Voyage Deluxe Convertible Car Seat is made by Britax and is basically the same shell as the Marathon. It is about $160 on-line, although it goes on sale often. The Evenflo Triumph Advance is a nice seat as well, at about $120ish, and it will last her until she is 3+.
As for your 4 year old, I agree with you that she should be in a 5 point harness as long as possible. I have my 4.5 year old in a Britax Regent and we really like it. I love that he will be in a 5 point harness until he is out of a car seat. It is just safer. I think this is your best bet. Sunshine Kids Radian 65 or 80 will work as well, as they have a 5 point harness to 65 or 80 lbs. But, the harness height is only 17" so the Regent is a better bet as that will last until she is 19".
I would stay away from the Eddie Bauer or Alpha Omega car seats. I had the Alpha Omega (my sister-in-law had the Eddie Bauer- they are basically the same seat made by the same company) and it broke after 3 years for both of us. It isn't worth it. The straps twisted all the time.
There is a really great car seat board out there where you can get reviews of all the car seats and ask questions and get specifics. It is www.car-seat.org. It's free to sign up, or you can just surf the boards and look for whatever seat you want to. I have learned so much after looking through there for my recent purchase of a Britax Boulevard and Recaro Signo.
Good luck!