On my iBook G4, I used to have TopSites.plist installed, which gave me 6 of my most used website pages. To click on Google or Thesaurus was an easy matter. I recently had to reboot with my original Mac OS X install discs, and when I got back to my Mac, the TopSites feature was gone. I don't know how to get it back.
Try doing a software update (second choice in the Apple menu) and see if you have an update available for Safari.
TopSites was introduced in Safari version 4, and the version of Safari that you're using is probably 3.x, considering that you're using a G4 and it probably shipped with Tiger. ;)