If your only connection is DSL, you truly won't have to worry about the lack of "/1000"
Your DSL is most likely slower than 8Mbit, which is slower than the lowest speed the ethernet card in the iMac supports. Some people have 100Mbit cable connections ( Like in Malmö, Sweden ) and there the 100Mbit mode of the iMac ethernet interface comes in handy. But unless you have a gigabit enabled computer at home that you are going to talk to, and a gigabit switch to connect the two computers together, you don't have to worry.
And gigabit to the home is not quite around the corner for most of humanity
