The easiest way is to replace the router with one that includes Wireless capability. You could also put an Airport card into the eMac and share it's existing connection, but that would mean leaving the eMac on whenever you wanted to use the Internet from your PowerBook. Lastly, you could buy a wireless access point and plug it into your existing router.
I take it from your "modem/router" comment that you are on ADSL user. There are a number of different solutions from companies such as Linksys, Netgear, Zyxel etc. if you decide to take the replacement route. I personally use Netgear and have no problems with it, but others will likely be able to recommend cheaper alternatives that provide the same functionality. The same companies also do wireless access points, which work out much cheaper.