The speakers inside a PC, laptop or monitor aren't what you want. So you need an external speaker that connects to the audio output of your PC. Could be anything from a cheap stero set (USD 10 to 15) to a full blown 5.1 home theater speaker set (USD 100 to 500).
The wattage of any set will be in the specifications.
Kees
I want to buy a powerful computer speaker , How can I know the extend of the sound is loud or weak .
Is it check the output watt of the speaker ???
Any other suggestion to jusify the loudness of the speaker ??

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic