It depends on how much of an audiophile you really are. For example, I have somewhat of a discerning ear when it comes to music, so I use a Santa Cruz sound card connected to a pair of Boston Acoustics BA790 Digital speakers. The rest of my family doesn't have quite the ear that I do, so they use the onboard sound on a Via KM133 board connected to some Altec Lansing ACS33 speakers. My sister told me that she can't really tell much of a difference between their setup and mine, but I certainly can. That said, the onboard sound definitely use more CPU cycles. I would try the PCI card first and see what you think. If you doesn't sound good enough, switch to the onboard sound and see what you think. If neither suffice, then you're probably in the same boat I am: get a Santa Cruz. 😉
Good luck!
Nick