I bought a cheap soundcard (x-fi extreme audio) that I use with crative T-20's for pc use, and also hook up to a kenwood av amp and tannoy speakers for movies/entertaining and I cant say theres any difference over onboard in quality per-se, ( I have a pretty moder motherboard ).
One big improvement it has made though, is that theres no noise interference that I used to get with the onboard, if i were to jack the volume up, I could hear what sounded like a 56k modem in the background when the music was quiet, maybe interference from fans or the hard drives i dunno - but thats completley gone now, sounds perfectly clean....so im happy as that was the main reason for me buying a dedicated sound card.
My advice would be to build your new rig first, then evaluate the sound - you may find you are pleasently surprised with modern onboard sound. If you do decide to upgrade I would get a decent set of speakers first, and then consider a soundcard if you still have issues.