Skylab did a review on the D5000, and he said they really needed an amp. Plus, when you are spending that much on headphones, why would you want to use them outside? Most high end headphones are really better suited to home use with a full "rig" of sorts.
As for the impedance thing, low impedance does not always mean that the headphones are easy to drive. The biggest example is then AKG K701: It has a relatively low impedance, but is one of the hardest dynamic headphones to drive.
Also, the HD650 has a really high impedance, and needs an amp.
I think the d2000 would be a good choice, however it will sound better with an amp, even a portable one.
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