I'm using a Dell Inspiron 580 and Windows 7.  I didn't order it with a sound card and I've been using a headset whenever I've needed to listen to a Youtube audio.

I'd like to add a pair of speakers, nothing really fancy, just something that will give decent sound.  Definitely less than $30.   I'm assuming I need "powered" speakers but I don't know if I need the ones that get their power just through the USB cable from the computer, or the ones that plug into an AC outlet. 

Any advice as to the type and specific model would be appreciated.  I also need to know if I will have to invest in a sound card too, or is the motherboard's integraged sound circuity sufficient.