OT: initrd / change root fs

[schnobs@babylon-kino.de]

Hi,

this clearly doesn't belong here... but I guess my chances
to find someone who knows about the subject are far better
here than anywhere else.

Like many others, I have an older machine serving as maqerading
host, fax-/dhcp-/nameserver etc. in a very small office. As is
the nature of these boxes, most of the time it has very little to do.
Now I wonder how I could move all the files that are needed for
daily use into RAM and spin down the disk.

One means that comes to mind would be a very big initrd;
however, this would also mean that every upgrade or install
would nevessarily be preceded by a reboot and followed by the
generation of a new initrd. I don't think that's worthwhile.

More preferrable: I'd like to change the root fs under all
running processes much in the same way as you can pull out a
tablecloth under the dishes.
Ideally, I'd like to end up with a script that copies the
existing files into the ramdisk and changes the root fs; this
way, I could simply switch back to the harddisk for my 'apt-get upgrade' and just as easily 
copy the upgraded system back into
RAM.

Now, before I start trying: is this possible at all?
Any hints, ideas or recommended readings that could protect
me from the worst frustration?

TIA,
Schnobs