I recently built an X4 955 PC with an ASUS M4A79T Deluxe. I installed the latest Realtek drivers and fired it up. The sound in games was pretty mediocre (skipped and missed sounds occasionally as well). Stuck my trusty Audigy 2 Value in from 2004 and it was much, much better. I'm still using XP and with the hardware acceleration, even with a new quad, I noticed a small improvement in FPS (a couple more in most games).
Realtek is fine for most users, but if you game or are an audiophile a dedicated sound card is a worthwhile purchase. Even my old cheapo is better than onboard.