Thanks for the suggestions all - I think I'm going to go with the O2. There is a decent deal on the classifieds for one now, although I was thinking about buying the components in a kit and building it myself. I think that would be a fun project.

I just ordered the SoundMAGIC HP100's and I'm looking for a decent amp to drive them that won't break the bank. The form factor will be desktop as the cans won't be leaving my desk. I currently have an Asus Xonar DX PCI-e card, and I figure that should be sufficient DAC for these headphones...