I recently purchased an Echo Indigo IO notebook soundcard (cardbus interface) and just hooked it up to my Sennheiser HD-600 headphones. It does a fantastic job driving the headphones with great detail, bass and overall sound quality. No need for a dedicated headphone amp with this soundcard. It also does a very good job with my Etymotic Research ER-6 earphones, but the HD-600s sound much better than the ER-6s. I have a pair of Grado Labs SR-225 on the way and will see how they perform on the card (the Grados require less potential to drive, but are more current hungry).
The card also does 24-bit/96kHz recording and comes with basic versions of professional recording software, just in case you want to crank out some albums too. I highly recommend it for hi-fi notebook listening or recording.
