Is your DSL line running over the phonelines or cable-tv?
If the phoneline, do you usually have some noice on the ordinary line when speaking?
Otherwise complain to your provider and get them to test the speed.
I also have an integrated ethernet port and it should still be a 10/100 Mbit port, (although far from that line speed
) and I really doubt you would have a bottleneck there, unless you have a defective ethernetport wich could be tested setting up a LAN and seeing with what speed you can transfere data over it.