Note that you can use a 64 bit kernel with debian on s390 in order to use more than 2 GB RAM, but there are a few applications that might not run in that environment.
If you are feeling spry, there are a couple of s390 cross-compile walkthroughs out there... notably http://pax.gt.owl.de/~higson/debian-s390/HOWTO-debian-s390-port.html
