The best way, and most simple is to get a reciever for your regular speakers, and any new reciever will have several possible inputs from a computer. This could range from a regular male to male connector to an optic cable. This would allow you to easily hook up anything to your speakers ranging from an ipod to your pc. Buying a new sound card can improve the quality of the sound but try it before you buy it. When setting it up, plug the male connector into your headphone jack or audio out and reduce your pc's volume levels especially wave, let your reciever do the amplification.
and just fyi, sound cards usually have about a 2 watt driver, you were correct in saying this is far from capable of powering speakers alone.