The latest version of Safari that runs on PowerPC (PPC)

The current version (iTunes 10) will run on a PowerPC machine but it has some small limitations (primarily it can not play High Definition video from the iTunes Store)

System Requirements:

Mac computer with an Intel, PowerPC G5 or G4 processor
Intel, PowerPC G5 or 1.0GHz PowerPC G4 or faster processor is required to play Standard Definition video from the iTunes Store
2.0GHz Intel Core 2 Duo or faster processor is required to play HD video, an iTunes LP, or iTunes Extras from the iTunes Store
Mac OS X version 10.5 or later

Source (iTunes Download Page)

I understand this is an older thread, but for the next person who comes along, I recommend taking a look at Safari Keyword Search. It is a Safari extension that adds exactly what you are looking for.