Lydia,
Yes, the onboard LAN did work when I first was building the box and when it was directly connected to a cable modem (from Astound Broadband (i think they used a webstar modem)) and unfortunately I never checked if it connected at 10 or 100. It merely worked and was fast so I did not question it.
Now I am using a BEFSR41 (linksys) router (and I tried hooking it up to the DSL modem as well numerous times) for a DSL connection. I cannot check if the onboard LAN works at the cable modem (as it did originally) due to distance from that location (sorry).
Basically when I hooked it up to the BEFSR41, it did not like it one bit. The router lights blinked. When I went into the driver settings (I tried this with both the upgraded and original Microsoft XP drivesr) and switched it from auto-negotiation to both 100mbs and 10mbs. Neither worked with the router or DSL. This would not work in linux either (many distributions, mostly red-hate based and debian based)
After trying endlessly to get it to work for 2 days or more, I just left the machine off and a few hours later I turned it on (and I believe it was either set to auto-negotiate or 10mbs mode in the driver (windows xp)) ... after 2 or 3 mins it connected to the internet on its own. This was recognized at 10mbs. I was very happy. I used the web without incident for some time. I rebooted -- the LAN no longer worked. I was unsucessful in getting it to connect another time.
Hopefully this will help.
This was using the T1.1 bios version. Athlon xp 3000+, sl-k890, corsair ram (some expensive crap), M-AUDIO revolution 7.1 + audigy 1 (which I had to remove for the PCI ethernet card!)... The thing is, I think it may have something to due with contention over the PCI bus. I removed the m-audio revolution but that did not help it connect. I reseted the cmos a few times etc... nothing worked. (Note, the internet worked on the cable modem initially with the m-audio). I inserted the audigy 1 into the motherboard along side the revolution 7.1 but did not test the internet (I had removed the cable). So whether the audigy had anything to do with it is up for grabs... after removing it and everything from the PCI bus, the problem still remains. Now I just placed a jumper cover on the enable/disable onbaord LAN jumper to kill the LAN.