Q: what would be the most recent upgrade versions for  an 'E-Mac'

If his model has an 867 MHz or faster CPU then he can upgrade to Leopard - 10.5.x. You can probably purchase a retail Leopard installer pack on eBay, Amazon, or Google.

For third-party software go to the publisher's website for upgrades. For other system upgrades use Software Update.

And, because you haven't told us anything about what you trying to upgrade, I don't see how we can help you further.

Only by upgrading the system software to Leopard can you upgrade Safari.

That model does not officially support Leopard, but many people have found workarounds.

Search Google for "upgrade 800 mhz G4 to Leopard" (without quotations) and you will find a number of different solutions.

There are workarounds if the 867MHz CPU is the only hangup...

http://sourceforge.net/projects/leopardassist/

http://lowendmac.com/osx/leopard/unsupported.html