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Hi Alex,
I used to use ISO 400 for 5 minute shots and ISO 800 for 3 minutes to minimise noise and depending on how good my guiding was. For short exposures you can push it to 1600 or more but you will introduce a lot of thermal noise, which defeats the purpose. I was using a 1000D and when I built a cooler box which dropped the sensor temp by 10 - 11 degrees, it made a LOT of difference to the result.
Not sure of the thermal or noise characteristics of the 400D, but have a play and see the difference you will get. ISO 400 was my sweet spot on long subs.
Cheers
Chris
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