[Terry B]

Part of the problem is the dynamic range of the image as well as the sky gloow you are getting.
Increasing the ISO increases the amplification in the camera but doesn't change the amount of light. there is an ideal ISO for these cameras that will give you the best dynamic range. I remember seeing a review (But cant find it at the moment) for a 40D that had ISO 800 as the sweet spot.

These cameras are 12 bit imagers so have a range of +/- 4096 for each pixel in RAW settings. I don't know what the well depth for the sensor is. This number is then multipled by a factor and this is the "gain" of the camera. This is what changes with the ISO rating. If the well depth is 30000 (as an example) and the gain is 0.25 then the total depth is 7500. this is higher than the dynamic range of 4096 for the image so data is lost.
try using a lower ISO and see if you can get a better dynamic range in your images.
Obviously moving to a dedicated astro imager would improve things as well.