Dear Steve,
Hi - I don't have an Atik 420, but do have an Atik 314 and 460ex.
Looking at the spec for the 420, it can reduce temperatures by 30 degrees.
In general, the cooler your camera, the less noise there is and the better your pictures. However, the bias and dark frames (if you need them) need to be taken at the same temperature.
For my Atik 460, I try and image at -10 degrees. I can generally achieve this temperature except in the height of summer, when I might need to go to -5 degrees. I could go colder in winter, but the noise is already pretty low at -10 and it means I just need dark and bias frames at -5 and -10, rather than trying to get some at -10, -20 etc.
For the Atik 314, the noise is already pretty low at 0 degrees, and I can always image at that temperature, so never try to go lower.
I would guess the Atik 420 is more like the Atik 460.
I hope this helps,
Colin