I recently hooked my PS3 up to my PC monitor too. This is how I did it; I think it will save you some money.
HDMI-DVI cable (HDMI one end, DVI the other)
AV component cable (comes with PS3)
RCA in 3.5 headphone out adaptor
3.5 headphone to 3.5 headphone adaptor (connects to above adaptor and the cable from your speakers)
That should be all you need.