Security Update 2011-004 (Leopard) - Ditto. Mac OS X 10.5.8 Combo Update - You might might have installed this already. Safari 5.0.6 for Leopard - Apparently the last version for PPC. If you've checked for updates through the Apple Menu > Software Update, and nothing is available, here are some links (not direct downloads) from Apple for both Safari and OS X 10.5.6: Sadly, as systems, OSs and their components age companies start dropping official support.
It usually is a good idea though to remain current with security updates to Safari, as with system updates. If Safari is still functional, it's not required to update it to keep using it.