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.
When you say it hangs, does it kernel-panic? If so, you can tell the
kernel to reboot after a delay instead of just sitting there.