Try to find an old card listed in this link:

That is the list of all cards which worked with RHL-6.0 10 years ago.

If this is critical, then you might be able to scrounge a card from the local computer thrift market. But remember, even if you fix this machine, you are running an OS for which innumerable security vulnerabilities were uncoverd in the intervening decade. You would not be wise to put this machine on an open link to the Internet without a firewall appliance protecting it.

Copying the driver over most likely won't work as it would have been compiled for your Suse box.
You should download the source code for your driver: http://support.3com.com/infodeli/too...uxdownload.htm

Before installing the drivers:
Use 'lsmod' to show what modules are installed and look for something along the lines of 3c509 and remove it using 'rmmod <3c509 module name>'

Then extract the driver source somewhere, configure, make and install/modprobe it...the instructions for this and further steps should be bundled with the source, but as tredegar mentioned ealier, those drivers were only tested on a later version of redhat to yours so you might have some dependency issues, but then again, you might not.