Measuring the number of decibles that your computer gives off should be fairly easy...
The way I'd do it is just to record some noise from my microphone input at a generic volume level, then analyze it with an audio analyzer. I dunno where you could find one, but fruity loops 3 might have a tool to tell you the decibel level of a clip or something.
Try a google for "decible measurement" or go to your nearest radio shack and ask for a decible measurement tool.
Remember, however, that the diff between 1 decible and 2 is not the same as 4 and 5. From 1 to 2, the sound intensity should increase 10 times. From 2 to 3, another ten times. So if you try to calculate the decibel level for your different components, you're gonna run into trouble.