Nah you dontneed to install car amps if you already have a pc speaker system.it has the amp built in.i just did this on my cab to get audio out the pc speakers when i have my xbox running.all you need to do is get a 3.5mm splitter cable,male to 2 female,a regular 3.5mm male to 3.5 mm male wire and the 3.5mm to rca splitter brian said to get.
Now plug the 3.5 to 3.5 splitters male end into the green port on the back of your pc sub,then plug the wire coming from your pc into one of the splitters female ends,now take the regular 3.5mm cable and plug it into the other open female jack coming out of the back of the sub,take the other end and plug it into the 3.5 to rca,now just plug in the rcas coming from your ps3 and your all set.
That is the simplest way to get the sound out without cutting or hacking anything.the only problem is sound will mix if both the devices are on at the same time,but thats not a big issue,obviously you can only use one at a time anyways.there are many ways to acomplish this,when i did it this i used a relay to automatically switch over audio signals,and made my own wiring harness.its not necessary the way descrubed above is fine,im just OCD.