If you use a car speaker amplifier (or multiple amplifiers, with a splitter or mixer) then it'll likely require some cable modification, as car amplifiers use pinch-type terminals for input sometimes. It'd be very easy, though, just cutting the RCAs off a 3.5mm/2xRCA cable and stripping the wires back.
As far as ohmages goes, that almost always depends entirely on the speakers, as amps can just cut back on power if a greater impedence speaker is used (and more power if it's lower). I'm no expert on car audio, but it's a pretty simple thing to do.
Why the hell you'd want to when there are perfectly good sounding stupidly bassy systems designed just for computers is beyond me, but hey - if the OP wants to do it...