hi
with the internal A/D converter(s) and 'breadboard' style wiring, I don't think you can do much better than the 'noisy' specs you give (i.e. 200, 199, 200, 201...).
Considering that the 5v range is represented by 1023 units, a unit of jitter is about .1% of the measurement, or 5mv... I think you will be hard pressed to get much better than about 1mv accuracy with this grade of chip, wiring, temperature control and fabrication.
That said, there is a tutorial in the Playground and her that show you how to hook up 63 sensors to the Arduino, and here:
You could also look into external high-precision A/D converters. There have been a few posts about these in the forum.
Daniel