[CentOS] Lan Kernel Problem

I'm building a Linux box to act as Proxy/Router/Firewall. I'm using CentOS 4.5, with an "old" motherboard (Asus A8V-X), and two Ethernet NIC, based on a realtek chip, that's widely supported under 2.4 and later kernel (the cards were functioning excellent in another PC whit Fedora Core 1). CentOS detects the on board LAN, but not the other two, in fact, knoppix 5.0.1 doesn't detect too (kernel 2.6.17), but, Knoppix 5.1.1 (kernel 2.6.19) detects all three cards. Do you have any idea why this behavior? Centos 5.0 detects all three too, but I don't now why, my firewall script (ipv4) doesn't work with this release.

Now, thank you very much! Best Regards.