thank you - as a Debian-newbie I wasn't aware of that repository.
Unfortunately the doc there is sparse and I still have problems:
I've enhanced my /etc/apt/sources.list:
deb http://ftp.de.debian.org/debian/ stable main contrib
deb-src http://ftp.de.debian.org/debian/ stable main contrib
deb http://security.debian.org/ stable/updates main
deb http://www.backports.org/debian sarge-backports main contrib
non-free
Then I've tried:
$ sudo apt-get -t sarge-backports install postgresql
Reading Package Lists... Done
Building Dependency Tree... Done
postgresql is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 57 not upgraded.
$sudo apt-get -t sarge-backports update postgresql
E: The update command takes no arguments