Before an upgrade is to be installed, it is best to backup your files as a precautionary measure.
A partition is not necessary. All a partition does is divide a HDD into two parts (called volumes) which act as if they were two independent HDDs.
If your intent is to install 10.6 (Snow Leopard), run a Time Machine update first and then install the new OS.
Partitioning a hard drive:
To resize (or delete) an existing partition without losing data, you will need to use, for example, iPartition: