Just speculating here, but it should work with your PCI RTL8139 chip. My RTL8139 is built-in on motherboard, but OS X recognizes it as a PCI device and it works with this driver. Give it a go, got nothing to loose and tons of internet pr0n to download....
On an unrelated note, I used this PCGenRTL8139Ethernet driver since OS X 10.4 days and popped it in when i first went to Leopard and it worked fine. However, when trying munky's boot method a few weeks ago, i used the opportunity to upgrade to this new driver specially designed for Leopard by some 'john stormdrake' avail from here
. Do NOT use this! (At least if you have same or similar NIC as me, RTL8101L)
Although the guy claims he made it specially for Leopard from the orByte's PCGenRTL8139 source, he sets it to Disable Flow Control
which causes problems once you download something for a long duration with many peers (eg. BitTorrent). My internet worked just fine at first for browsing for a few days, but once I put a 6 GB torrent to download, I started seeing "TX outbound packet transfer error" like crazy in my system.log and "corrupt chunks" errors from my BT program. I tried all how, all ways, and user suggestions given to fix it, but nothing worked. Many people decided to get a Wii USB Ethernet converter to replace onboard RTL8139 NIC. But in the end I came back to this ancient PCGenRTL8139Ethernet KEXT and my 6GB downloaded great without a single error in system.log or my torrent program.
It may be years old and unsupported, and not officially for Leopard, but I can deff say it solved my corrupt packet errors.