In digital astrophotography, the key is to capture more signal to overcome the noise. You know noise when you see it and I am not going into a deep technical explanation of how it originates and what it is, but I know it when I see it.
I started astrophotography with the 350D and saw noise, especially color noise, that I knew had to be dealt with by traditional noise reduction methods. Those methods involve subtracting dark frames from the light frames.
Later I began using the Canon 450D and noticed a huge difference in noise. There was hardly any to speak of, even after moderate stretching of the data. This led me to experiment with a new way of optimizing the data without utilizing dark frame subtraction. That new technique is detailed in the archived webinar under the "digital photography education" tab.
Now comes the 500D and thankfully dedicated astronomers like Gary Honis put together bench testing of cameras so we know how they will perform. The 500D has even lower noise than the 450D and offers those ultra-high levels of ISO up to 6400. Now this will be exciting and is why I am ordering one soon.
Here is a link to Gary's test. This is a new day in DSLR photography, one that I hope will see the end to autoguiding as well as dark frame subtraction. Enjoy the new day!