You can definitely connect OS 9 and OS X through Ethernet. I did a major backup of my old Power Mac 9600 (OS 9.1) to my Mac Mini (10.4) just last week. It was, however, a little tricky. It seems that OS X can't load OS 9's shared folders over AppleTalk (at least Tiger can't). However, it can mount them over IP, and OS 9 can access OS X's over AppleTalk just fine (go figure).
I find it easier to use OS 9 to mount my OS X drives than vice-versa, for the simple reason that OS 9 tends to crash a lot when you mount its drives on another machine (YMMV, of course). Here's what you need to do:
In OS X:
1. In System Preferences:Network, go to the built-in Ethernet options, and make AppleTalk active. Be sure to click the "Apply Now" button after checking the box.
2. In System Preferences:Sharing, enable Personal File Sharing. If you use OS X's firewall, go to the Firewall tab and choose to allow Personal File Sharing.
In OS 9:
1. Turn on AppleTalk. This can be done A) Through the control strip, or B) In the TCP/IP control panel (or is it the Internet control panel? I don't have an OS 9 system on hand atm, but it's something like that).
2. Open Network Browser (by default it's under the Apple menu; if not, just use Sherlock to find it). Your iMac should appear in the list (in may be buried under "AppleTalk" or "local network"). Double-click it, and log in using your OS X username and password. If you log in as an admin user, you'll be able to mount every volume from your OS X machine. If you log in as a non-admin user, you'll only get that user's home folder. Either way, you should be able to copy the files without a hitch.
I forget the exact instructions on doing it the reverse (mounting your OS 9 drive in OS X), although I did it just last week. If necessary, I can check again and write up instructions for you.