I have a similar problem. Also using Redhat 9 and have a sis900 embedded LAN. The sis appears to be working fine, and I've tried everything that I found on various forums regarding this.
I can enable the eth0 connection, which supposedly patches me into a splitter. I've set the connection with all the same options that work on my XP partition. When I try to set up ppoe though, it fails to initialise.
Also, when I try to run a test on the eth0, or maybe limit its speed to 10 Mb, I get an answer that it can't find the device. Any ideas?