When I installed Ubuntu for the first time, I named my system to reflect the os name (gutsy) and the architecture (64 bit). However, whenever there were system upgrades, the name got irrelevant. For example, when I upgraded to Hardy, the name still remained stuck to the last version of OS.
Therefore, I now thought of renaming my system to separate it from the os versions. This is so very simple to achieve in Ubuntu -
just edit the file /etc/hostname (owned by root, therefore use sudo). Put in the new name, and restart the system.
However, the consequences of this name change will have to be reflected across the network (if you have one). Hostname files on all other systems or dns entries will have to be updated to reflect the new system name.