I've not tried astrophotography, but the general rule with digital cameras seems to be that you're better off using longer exposures at lower ISOs than shorter ones at higher ISOs. Longer exposures increase your chances of seeing individual "hot" pixels, but increasing the ISO amplifies the background noise so you get more "hot" pixels.
You can see this type of effect in the DPReview D1h night exposure examples: 15 seconds at ISO 200 much is cleaner than 5 seconds at ISO 800. Interestingly, though, there seems to be a trade-off for very long exposures with 30 seconds at ISO 200 being about the same as 5 seconds at ISO 800.
Keeping the camera cool (I mean letting it adjust to the ambient temperature, not putting it in the fridge ) seems to help reduce noise.
P.S. Looking forward to seeing your photos of the planets... what sort of telescope were you using?