In most mobos today, you can overclock the cpu and agp bus independently of the pci bus so while you oc the cpu, ram and vidcard, you can leave the pci bus at 66 mhz preventing pci devices from becoming unstable. Soundcards have their own clocks and partially derive their timing from the pci clock so any attempt with oc'ing the pci clock can destabilize that timing. In extreme cases, you can possibly fry the card but that is rare. On older mobos, oc'ing the cpu bus also meant oc'ing the pci bus so be warned.
Some mobo/cpu/ram combinations are more tolerant of overclocking than others. Gentle overclocks can be done using stock voltage, but In more radical cases, increasing cpu and ram voltage is necessary in order to achieve a stable overclock configuration. The side effect of this increased voltage is more heat thereby requiring better cooling solutions and this usually means more powerful, noisier fans or watercooling. I'd rather have a quiet pc so I don't overclock. Years of listening to the whine of a Volcano 6+ have made me appreciate quiet fans and PSUs.
