The nForce drivers to get your nVidia LAN working.
You need to use that 3Com file to get your 3Com working.
I doubt you'd notice any speed differences between the 2 LANs ports. The nVidia one is known to have lower CPU utilization.
Also, I recommend against using the nForce drivers directly from nVidia. Asus uses a custom MCP-T chip and there are some things, notably the sound, that seems to mess up using the drivers from nVidia. So use the nForce drivers from Asus.