Max it out.
Install a larger HD
Install OS X.3.9
That's about it. Processor is not upgradeable
P
Hey, i just added more ram to my imac, its total went from 256 to 640. This has helped the overall performance, and i was wondering what other upgrades i could do. I have a g3 imac 600 mhz (graphite). thanks.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic