Funnily enough, I have never managed to get any Realtek product (ethernet, wifi) to work reliably in a Mac. I've tried a 10/100 PCI ethernet card with patched drivers on Mac OS 9.1, with no success, and in the absence of any diagnostic software I can't recommend them.

Retroman: Asante 10/100 PCI cards appear on eBay UK every month or so. They work.