Safari 4.1.3 is the final version to run on Tiger. Safari 5 will only run on Leopard 10.5, Safari 6 only on Snow leopard upwards.
On a PPC Mac you can not upgrade further than Leopard 10.5. This is no longer sold by Apple so you will have to look on Amazon or eBay for a black retail install disk (grey update disks will not work). Be prepared to pay anything up to $200 for Leopard!
Safari for Tiger is old and hasn't been updated for years.
Get a modern browser for Tiger. I use TenFoxFour, an upgrade from Firefox. It's free.