Firstly connect the hdmi/dvi cable to the ps3 and the monitor.
If your using a stereo connect the av multi cable to the ps3 and then connect the red and white rca plugs to the aux or any rca input on the stereo and switch the input to aux or the input you connected too.
If your using pc speaker’s connect the rca / jack cable to the white and red connector’s from the ps3 and the the jack to jack interconnector to the jack cables from the ps3 and the pc speakers. They can only connect one way so don’t worry it will be simple once you try. In any case you will not be useing the yellow rca as this is a vieo cable so just let it hang lose.
If you’re using a stereo but don’t have a aux port or free input then use the pc speaker’s method but connect the rca / jack to the mic input on the stereo.
Now turn on the ps3 by pressing and holding your finger on the ps3 power button until it beeps once and then let go. If the ps3 beeps more than once then turn it off and try again. The ps3 will then be displayed on the monitor and will say hdmi detected do you wish to use this now’ say yes. Its most likely that at this point the ps3 will try to output to 1080p witch your monitor does not support don’t wory though just wait 30 seconds and the ps3 will automatically revert to 576i.
Now log on and go to the display option's menu and select hdmi as your output but select custom resolution. The next screen will ask you to select what resolutions you want. Tick 720 and make sure 1080p and 1080i are not ticked then continue. Your screen will now be set to 720 witch is fine for a 20inch screen. You will then be asked if you want to set the audio option's say yes and on the next screen select scart/av multi out and your done.
If you have any trouble setting this up let me know and ill help as best I can. : )
please note that if your using a av multi as an audio output that you will only get 2.1 sound. For 5.1 you will need to go down the optical road witch will require more expensive equiptment