OWC is a Mac-oriented vendor, and I see that they sell four USB devices for networking wirelessly. Go to macsales.com and search on "USB wireless" to see them. However, Mac OS X 10.2 is your limiting factor. They all require 10.3.9 or later (for the special driver).
Instead, you may want to try an Ethernet wireless bridge. These devices plug into the Ethernet port, and from the iMac side, it looks like a regular wired connection. No drivers needed. Here's one example.
http://eshop.macsales.com/item/Linksys/WET54G/
They are more expensive than the USB adapters.
For the router, I have an Airport Extreme base station from Apple. It has gigabit Ethernet ports in addition to the wireless. The setup and management software, Airport Utility, is excellent. You can attach an USB external drive and make it a NAS (network attached storage). The more recent Time Capsule is basically an Airport Extreme base station with a built-in hard drive. My Airport Extreme has been trouble-free, so I can recommend it as a good choice.