filesystem for initrd

[Art Boulatov]

Hi,

I'm in the process of creating a custom "system partition"
for out Linux servers, which is actually an initial ramdisk,
coming from hd or network on boot
to load necessary drivers and perform important checks
before the real filesystems get mounted,
and I did not find any info on
what filesystems can I use
for initrd, are there any restrictions?
Mostly interested in cramfs,
due to it's compression.

Could anybody help me work this out or point to the right place for more 
info?

Thanks a lot,
Art.