questionswhat cable do i need ?

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You need one like this:
http://www.amazon.com/RiteAV-3-5mm-Stereo-Male-Cable/dp/B000V0G2C4/ref=sr_1_1?s=electronics&ie=UTF8&qid=1324316247&sr=1-1

11:36am Link corrected.

I don't know what connections the sound bar has, but if you say RCA, I'd assume it has red/white plug inputs.

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@omnichad: You are right. You need the one that says stereo!

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The answer to this question actually depends on your soundbar and on your computers soundcard hardware.

Most SoundBars have at the least a digital in, and a stereo in (red/white RCA) Most modern computers have both headphone out, and a digital out of some sorts as well, most specifically an optical digital out (TOSlink) which would be the optimal connection you could give your soundbar from your computer. If you don't have TOSlink ports, then 1/8 inch stereo headphone to RCA stereo out is the answer. Both cables can be found for $5 to $10 shipped, or from a local electronics store (Radio shack) or hardware store (home depot, menards, lowes all have a home theater wiring section)

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Ok sounds like some more deails are in order though I think the question has been answered.

VIZIO High Definition Sound Bar, VSB200 is what I bought it has 2 analog inputs and a digitail one which I beleive is the SPDIF.

I think I have a port like that on my machine it has a little gray door over the hole and glows red on the inside ?

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@djbowman: Yup, the little grey door with the red glow is a Toslink (aka, but not entirely the same) SPDIF (Sony-Phillips Digital InterFace). Buy a little digital optical cable from Monoprice and you are in). Any of these will do:
http://www.monoprice.com/products/subdepartment.asp?c_id=102&cp_id=10229

Note: SPDIF is the transmission protocol, and it can go either over the digital optical, or digital coaxial. I confused myself here. Some soundbars can do either, and some soundcards have both as well.