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Sound Card 
This card is needed if you want to hear anything from the computer, unless you have sound included on the motherboard which is becoming more common these days. However, having sound included on the motherboard is kind of like having a stock stereo in a car, it does the job but doesn't blow you away. If you enjoy listening to music at higher than normal volume then your best option would be to invest in a Soundblaster Live card.
The Soundblaster is made by Creative and is the leader in sound cards by far. They have been around for a while and the prices are very reasonable so it's definitely worth spending a few bucks for. Take my advice and buy the Soundblaster Live Value rather than any of the "fancy packages" that have software you won't need.
Cost
The sound card will run from $25 to $75.