Are you installing from a CD? If so, most likely you are installing Debian Woody. Normally its install uses the 2.2.2x (I think it may be 20) kernel which knows nothing of the nForce2 chipset nor its 3Com NIC. Even if you use the bf2.4 kernel, I don't think the 3Com would be supported as it is 2.4.18. The nForce2 did not really get supported until 2.4.21 (various pre's added more functionality). You need a much more current kernel than what comes with the original Debian the 3Com NIC and the rest of the nForce2. I think the 3c59x driver was updated in 2.4.21-pre3 to support the PCI ID of the 3Com NIC.
You could have grabbed the nForce drivers from nVidia and got the nvnet driver working on the nVidia MAC. It is a small download which you should be able to transport to your A7N8X DX machine via floppy. Once you have network connectivity, you could download the latest 2.4.21-rc2 kernel and recompile. Then you should be set.