-The CAT5 cable you have, I assume it is a straight thru cable. Look at the cable colors at each end by turning the ends over so the tab is on the bottom. They should look the same. If they are different, you have a crossover cable.
-If a straight cable, get a ethernet hub, switch or router and another straight cable and plug them into each computer and the hub...
On the iMac running 8.5:
-Turn on File sharing in the Sharing Control panel. Enter name and password for the computer. Close the panel
-In the Finder, select the 6GB drive icon and goto File menu, Get Info/Sharing and select to Share this drive. Close the panel.
-The iMac may take a minute to start file sharing. The Control Strip will say if it is done opening. Open the TCP/IP control panel and record the IP address. It should be something like 192.168.1.2. If it is 169.x.x.x you have a problem.
On the new computer running OS X:
-Open System preferences and select the File Sharing pane. Click 'Personal File Sharing'. Close the panel.
-In the Finder, select the Go Menu, then enter the IP address of the iMac. You should get a login window. Enter the Name and password you entered for the iMac. The iMac drive should mount. You are done. Copy at will.