I would actually recommend against the Alpha Omega, as it is only useable as a harnessed seat until 40 lbs, and the top harness slots are no higher than the Comfort Sport's.
Ok, here is what I would do
Spendy version: a Britax Regent, a Britax Marathon and keep Newbabe in the Comfort Sports (assuming they are 5-point harnesses) until he/she hits the rear-facing weight limit of 30 lbs. Then turn Newbabe FF in the Comfort Sport, or switch him/her to the Marathon, DS to the Regent and DD to a booster. Or hey, this is the spendy version, get Newbabe a Marathon too.
Mid-range version: Get a couple of Fisher Price Safe Voyage Deluxes. they are $179 here:
http://www.burlingtoncoatfactory.com...s_baby_depot=1
These are stripped-down Marathons, and will last your kids until 55 lbs or when they gain 4" more in torso height that their current seats. Remember, most kids at this age put on a lot of height in their legs, not torsos. So when your DD outgrows the 55 lb limit, pass this down to Newbabe (In a Comfort sport or something) and get her a booster then.
Budget option: a Cosco/Safety 1st Apexes here:
here with a couple caveats: these aren't as nice to use as the other seats suggested, and to use them as boosters you *must* have headrests in the back seats of your car.
Really budget method: Graco Turbobooster for DD, Evenflo Bolero for DS (it converts to a better booster at 40 lbs than most combos) and a Cosco Touriva/Scenera for Newbabe. This might fit 3-across in your current vehicle saving you a fair chunk of change.
I would feel ok about leaving the newbabe in the Comfort sports, especially rear-facing. Rear-facing is safest, and I honestly believe that proper installation and use is a bigger indicator of safety for a car seat than how much was spent. That said, the Britaxes are super duper easy to use, and therefore often safer
Also, depending on the size of your car you may be able to fit 3 across, especially if newbabe is in a fairly narrow seat. You may want to try something like 2 Safe Voyages and a Cosco Touriva in your car, and see if they all fit.
Lynn6, the harnessed limit for your CarGo is 40 lbs, or when your child's shoulders are above the top harness slots.