give me up to 50 watts per speaker.
I was thinking I might be able to use a car audio amp that would take 12volts from my computers PSU.
2x 50 watts (100 watts total) at 100% efficiency at 12 volts is 8 amps of draw. most amps are only about 80% efficient. so bump that up to about 10 amps. add into that that most 12 volt rails arent regulated and you will likely see somewhere in the neigborhood of 11 volts once you load it down... so throw in another ampere for a total of 11 amps.
You are going to have a tough time finding an acceptable power supply to feed it. i would also not run it off the same power supply as a computer regardless of how powerful the rail is. The voltage drops will play havoc on your system.
This is why home stereos amps have huge ass transformers in them. gives you lots of nice clean power.
check out local pawnshops and get yourself a nice older hitachi amp. you'll get probably 250 watts and 4 channels for next to nothing. and because it's older, it would be easy to unsolder the volume pot and mount it someplace else like the back or side if you wish