Quality headphone playback on a notebook computer?

I'm in the market for a new notebook PC, and I want something capable of high-quality music playback, through headphones... In my (somewhat limited) experience, CD playback sounds like crap when run through the built-in headphone outputs of notebook computers. (Apple Powerbook G4, Sony VIAO) Even marginal-quality MP3s played on my inexpensive iRiver MP3 player, put the notebook's CD playback to shame.

Are there any notebooks out there with acceptably good built-in headphone outputs? If not, what are the best options for headphone users? PCMCIA cards? Portable, external, USB sound cards or DACs (that don't require an AC outlet)?

I'm interested in CD, MP3, and DVD playback. Would an external DAC (like the "Waveterminal U24" that I've read about on this forum) prohibit playback of compressed music (MP3s), DVDs, video games, system sounds, or any other computer audio that I might use?

Right now, I'm considering the 'Soundblaster Audigy2 Z5' PCMCIA card. But, I'm somewhat biased against Creative, so I'd prefer to go with another option if there are better (affordable) alternatives. I'm looking for something that DOES NOT plug in to the wall, but can run off laptop battery power, and still provide enough juice to adequately power my headphones. (Sennheiser HD650, and Sony MDR-EX71SL earbuds)

Any help, advice, or recommendations will be greatly appreciated! Thanks.