Unless its a good bright object ISO800 for me. Its the closest setting for unity gain (thought with the 20D it really splits the ISO at 1200, but being modded I'll still use ISO800). For me its a case of multiple images with higher ISO to improve the quality and S/N ratio rather than more -e/ADU to reduce the noise. (
http://www.clarkvision.com/articles/...tml#unity_gain)
If you've not taken the time to read the Clarkvision site I'd recommend it for those that would like to know a bit more about the inner machinations of DSLR chips. Slip down to part 4 and check out the links
http://www.clarkvision.com/articles/index.html#part_4