IPNetrouter is a great utility and will do exactly what you are looking for. The website to get more info and download it is www.sustworks.com. All you need to do is install and set-up IPNetrouter (be sure to find the installation guide on the website and print it, especially if your cable modem uses DHCP) and then connect your computers together via ethernet. Your computer would act as the Router to the iMac and both of you can then access the internet at the same time via the same IP.
As a side note though, since this was a problem I encountered, You will NOT be able to play most TCP/IP Based Net Games on the iMac as it will have an INTERNAL IP assigned to it. The work around for this is to use the port forwarding features of IPNetrouter, but if you have no experience with this sort of thing, you might have alot of difficulties.. So... If gaming is your plan, be sure to realize that you will not be able to play Net Games on the iMac without some modifications. You will only be able to play them on the Computer that is directly connected to the net.
Hope that helps
[This message has been edited by b00mer (edited 04-15-99).]