I couldn't help chiming in. For extensive information on which built-in notebook soundcards have decent sounds, check out head-fi.org. That being said, IBM T-series actually has pretty decent built-in soundcards from what I have read. Nonetheless, if you want more, you should buy an external soundcard such as the Extigy or the M-Audio Sonica.
I have never had any external soundcards myself. However, I have products from both Creative/M-Audio (the Audigy 2 Platinum and M-Audio Revolution 7.1) installed on the same desktop. For music, the M-Audio Revolution beats Audigy 2 hands down. If the Sonica is anything like the Revolution, then it's really a worthy solution for the notebook. Nonetheless, due to its being powered by USB, don't expect it to power a pair of AKG 501 or Sennheiser 600 earphones directly.
An alternative would be a PCCard called Echo Indigo. Again, I don't really know how good this solution sounds in person.