@Paul2
Yes, you are right. The biggest load the motors can pull is 5A each. However, as you are not(hopefully) planning to hold them from turning, they will never pull this much more than for an very short moment. Assuming then you are gonna use them in a balancing robot, which weight isn't of to big dimensions.
You don't say what type of battery you are gonne use, Nicd/NiMh or LiPo. I have always used LiPo for my projects, as they deliver very stable voltage as they don't drop so much as they get charged out, and they are quick to recharge. Drawback is that they can not be completely emptied, as they then will be damaged.
I have no experience with NiCd/NiMh batteries, but what you are gonna look for on a LiPo-battery is a rating "C". It says many amperes it is able to deliver, respective to it's capacity. For example, if the battery you are reffering to is rated with 10C, it is able to deliver 3800mah * 10 = 38A continusly. Usually the battery also got a peak-rating, wich works the same way only that this current can only be delivered for a short amount of time.
If you are new to these kind of batteries it is worth noting that the 2-cell LiPo(2S) is 7.4V and 3-cell(3S) is 11.1V. However, fully charged they measure about respecitve 8.4V and 12.6V, so the last works great on 12V motors.
On my robot a use a battery similar to this one for driving the motors:
http://hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=9163
And something like this one to supply the Arduino, Xbee etc.
http://hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=9170
The reason I'm using two separate battrypack for driving the motors and the electronics, is to try to avoid noise. The negative pole for both batteries is connected through a 100K resisitor to provide common "ground". Don't know if the motordrivers produces so much noise that this is necessary, though..
Well, good luck Paul2, and merry christmas to you all!
Jon, Norway.