ISO-400 worked the best for me, between 5 minutes and 10 minute exposures. That was with the 300D and the 350D.
When I got the 40D, I took a walk on the wild side just before I got rid of it and started shooting at ISO-800 with 10-minute exposures and was able to get this type of image with it:
http://members.optusnet.com.au/mrozy..._NGC_4945.html
With the 5D Mark II, ISO-1600 was my default.
Dark skies help.
H