My housemate's used his powerbook to get a PC online like this. Wireless router --wireless--> powerbook (or in your case ibook) --ethernet--> PC (or in your case eMac).
He just used a standard ethernet cable to do this, not a crossover cable.
You may have to use a bit of poetic license with these instructions, and I'm using OS X 10.3.3, so it may be different for other OS versions. Plug the ethernet cable into your ibook and your emac, then go into system preferences on the ibook. Go to sharing, then go to the internet tab. Select, "Share your connection from: Airport", and then To computers using: "Built-in Ethernet". Then click Start.
On the Emac go to System Preferences -> Network, and select Built-in Ethernet. I think it will work using DHCP. That's about it I think. You might have to play with it a bit to get it working properly. Good luck!