I figure on using integrated sound too on my next build since I'm not too picky on that. If the overall system performs well (if you have a system with good specs), the resources used to process sound is not a big factor at all. I don't even use speakers, lol, except for the sound that comes through my laptop.
Integrated sound is fine for most applications. But a dedicated sound card can make quite a bit of difference. We recently put an Audigy2 in my kids machine which was running Abit's AudioMax 7.1 integrated sound. The difference is amazing. Not only is the sound clearer, but the surround effects in gaming are what really stand out. You can now hear things behind and to the sides whereas before, you couldn't discern what direction the sound was coming from.
I will always buy a sound card. Onboard sound makes some of my N64 games choppy. I have a SB Live in one of them and the difference is amazing. I often try to get out of onboard.
I don't see how it can make that much of a difference if you have crappy speakers. I think at some point I'll get some better speakers, and then getting a soundcard will be worth it.