Second the comment about making a full backup of your existing install first (use
CarbonCopyCloner v2.3) to an external drive before you do the install of 10.5, just in case things don't go quite right.
Note that some programs you may have installed will not work with 10.5, so if you have things like MainMenu, MenuMeters, SharePoints, Little Snitch or Diablotin installed, you will need to un-install them before you upgrade.