confused with speakers?

[diggidy]

Hi,

I have a samsung 22" LCD HD monitor in my room, and bought the VGA cable for my xbox360 so I could play it on it. Everything is working great except for the sound...

My problem is I want to be able to play sound for my computer, but when i'm playing my xbox have the sound for that. I'm great with computer but have no clue what i'm doing when it comes to audio.

I bought a LG LFD790 home theater system thinking it could work but have had no luck. Will this work on my computer and my 360? If not could you recommend any other method of getting the sound to work, preferably under 200$?

Lastly, I have a 3-5 year old dell computer will the standard green and blue inputs on the back, but don't know what sound card it has.

Thanks for any help you have to give