B.W.
You shouldn't use the bunting bags (Like the JJ Cole Bundle ME) anyway. They go behind the child, and they go around the straps, and the impair hte way the carseat works, and will actually void your warranty if you get into an accident and the seat fails, aftermarket prodcuts (those that don't come WITH the seat) are a big no-no. Basically you don't want to use anything fluffy, anything that goes behind the child, around the straps. All you want to put your child in is light layers, and a fleece top. So a onesie, a shirt, and a fleece jacket. Let her wear her jacket to the car, remove it, and then put her in the carseat. She wont' freeze or catch cold.
If you wear a puffy jacket or those bunting bags, it holds the baby away from teh carseat, and the straps away from the baby. A way to tell if the jacket or bunting bag is too fluffy, put your child in his winter jacket, and buckle him into his carseat. tighten the straps to where you think they are tight/safe with the jacket on, then unbuckle without loosening the straps, and take your child out, and remove his jacket. Then put him back in the carseat nad hook the belts. You'll see how loose it is,thats how loose the straps would be in an accident should your child be wearning taht fluffy jacket. THe jacket will compress in an accident and make those straps that loose. Your child will be just as warm, if not warmer, wearing light layers.
Again, no fluffy winter jackets, no bunting bags. Call Graco, call Britax, they will ALL tell you do NOT use bunting bags, like the JJ Cole Bundle me, or fluffy jackets. However you can use the stretchy ones that go over the top of the carseat, like a shower cap. I'll link you to a carseat board, please go there with any questions. Its owned a CPST (Certified Child Safety Technician) and the board is full of info! http://z3.invisionfree.com/Carseats/index.php
Also, I'll ditto the poster who said skip the roundabout and go with the Marathon. Its only another $50, but you'll get at least 2-3 more years use out of it before your child outgrows it. A Roundabout is a compact seat, will only harness to 40lbs, and will probably be outgrown in height before weight. A Marathon will harness to 65lbs and will fit most 6 year olds... my 5 1/2 year old, 45lb, 48'' child still fits in this seat.
Also, Britax Marathon will rear face to 33lbs. You want to keep your child rear facing as long as possible! The 'law' is 1 year and 20lbs, however research now shows that is much safer to keep your child rear facing until they reach the 33/35lb weight limit rear facing on thier convertible carseat. The AAP is recommending rear facing to 33lbs and 2 years old. WHy? Because child spine does not fuse until they are closer to 3. If you forward face your child before, and even a low speed collision could cause thier spine to stretch too far, causing parilyzation or death, or internal decapitation. 1 year and 20lbs is the BARE MINIMUM, your child is owrth more than the bare minimum, so please keep her rear facing as long as possible!
So again, no bunting bags/Bundle Me's, no fluffy winter jackets, stay rear facing as long as possible, and think about getting the Marathon instead of the Roundabout. :) And check out the carseat board I linked above, its very informative.