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You can do a search on "Astro Stacking" or "Astro Stacking Theory". You'll find that stacking only a few images, really has little effect on the signal to noise. It's not until you get to larger stacks that you begin to see the difference. The S/N increases by the square root of the number of images. So doubling the number of exposures only increases the S/N by 1.4 (square root of 2). Here's a quick overview, from the site of the program that David recommended:
and a good How-to:
and another good stacking discussion:
But there are much more rigorous discussions, if you're interested:
It also seems that the newer the camera, the better the noise is handled. Although I don't shoot much with a DSLR (I use a dedicated Astro CCD - currently), the difference in my 30D to my 7D is quite amazing. Older cameras work fine and can produce good results, but the higher the noise (lower S/N), the more stacking will help your images.
You might also want to look and and read about calibration. This, along with stacking is the best way to improve the quality of the images. Just be aware that there is no magical substitute for open shutter time, be it in many short exposures, or fewer longer ones.
Hope this is useful.