Today many motherboards come with built in sound that is fairly good. This is exactly what I have in my computer. I would never recommend a motherboard that has built in graphic card, however if you are not into making music or anything similar, and are just using your computer for everyday things, you can go ahead and buy a motherboard with a built in sound. That will save you some money and provide you with the very acceptable sound.

But please be aware that built in sound can take up to 20% of computer CPU utilization. If you are using your computer for playing games, even the low end sound card will greatly improve your gamin experience.

BUDGET PC

If you want to buy a sound card as a separate part, (recommended if you are into making music and/or playing games) Creative Labs Sound Blaster Live! 5.1 is a good and cheap choice for a budget PC.

 
Creative Labs Sound Blaster Live! 5.1 PCI Sound Card

Creative Labs Sound Blaster Live! 5.1 PCI Sound Card is probably everything you need for a good sound and it is the best deal right now. You won't need this card if you have a good built in sound on your motherboard (unless you play latest games). However, some of the older motherboards have bad quality built in sound cards, which could give you somewhat scratchy sound. If you have this problem, Creative Labs Sound Blaster Live! 5.1 is a good alternative.

BUSINESS PC
 

Any business PC should require no more than an integrated sound card. As you motherboard will probably provide an integrated sound (most of the new ones do) you won't need to worry about the sound card in this case. The only time you will want to get a sound card for a business PC is if your integrated sound is terrible and produces scratchy sound. However, this is very unlikely to happen with the new motherboards.