I've just done almost exactly the same thing as you are contemplating (upgraded to sarge, then from 2.2.20idepci to 2.6.8-3-386). I used aptitude to install the new kernel and seemingly everything went well, but when I rebooted, it gets to the boot message "Starting periodic command scheduler: cron" and then it hangs. I am trying to figure this one out right now. My advice is to make absolutely sure you create a boot disk using your old 2.2.20, otherwise you might be screwed; come to think of it, if the same thing happens to you, you might be screwed anyway as you're doing things remotely.- is it worth an upgrade ? - is it as simple as an apt-get install kernel-image-xxx ?
Regards, Dave Whelan.