You're not going to find an inexpensive speaker set with true 5.1 surround. You'll either have to purchase a receiver with an optical input and speakers separately, or else buy a set of speakers that has an optical input and Dolby/DTS decoding built-in, such as the Logitech Z-5500 set. The Logitech Z-5500 used to be sold for ~$175 but computer speaker competition has died off in the past couple years and now it runs for $400. Perhaps you could find a good deal on a used set.
I would not recommend the splitter that zjh2011 posted. It only takes the stereo signal from the red/white RCA cables (which CANNOT output a surround signal) and just mirrors it to all of your speakers, i.e. your rear speakers will be outputting the same sounds as your front speakers.
I hooked these speakers up to my xbox like stated above I used the hdmi cord and ran it to my tv then i also plugged in the regular tv cable to the xbox and just ran the red and yellow audio cords to the speakers. It sounds excellent!! It DOES give you surround sound, I have played this with many games but with Call of Duty World at War with Zombies and it sounds incredible! Just as good as my friends $800 surround sound set.
As far as I can tell, the cheapest options to have this work with the PS3 with true surround sound, would be something like this: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=350193812906&ssPageName=STRK:MEWAX:IT - With this product, you'd have to set your PS3 audio to output HDMI LPCM 5.1 44/48khz Or, if you buy a Tritton AXPro 5.1 Headset, it would come with a Dolby decoder that can output to these speakers.