If you're setting up a multi-user workstation, Debian suggests separating /, /tmp, /var, /usr, and /home. I would think this would be even more true for a PC intended to be used as a server. Debian's default partition sizes, tho, are designed around the intention that the PC should be used in a production environment where installed packages will remain rather static. Some of them tend to get overfilled on a machine that gets lots of upgrades. Over time, I have settled on this partition scheme for my 80 GB, 256 MB RAM, 'hobbyist' system.
debian:~$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/hdb1 897M 206M 644M 25% /
tmpfs 219M 0 219M 0% /dev/shm
/dev/hdb9 63G 5.2G 55G 9% /home
/dev/hdb8 897M 17M 833M 2% /tmp
/dev/hdb5 4.6G 2.2G 2.3G 50% /usr
/dev/hdb6 3.7G 1003M 2.5G 29% /var
I just recently reinstalled, and haven't moved most of my data back to /home.