Its usually static that causes sound glitches with the built in sound chips (ac'97 chips on motherboards, namely)
If you overclock it will certainly be the cause, but like big b said, your best off with a pci card.
The sb 5.1 digital is best for normal and gaming use.
If you have quite a few pci cards you might be best off with something similar to but not a sound blaster. I think they use up two irq spaces and can (not always) cause a few minor problems too. But if you just have say a modem and sound blaster pci cards out of say 6 spaces you will be able to sort the irq conflict out.
Whatever you do dont go spending loads on an Audigy 2 or summit, they are good but you are spending nearly 3 times the money that a 5.1 Digital costs for 6.1, the extra eax feature (which aint all that good) and for 24bit sound.
The 24bit sound is great but not worth the extra money, but the rest is a blag IMO.