If you're using 2-way car speakers, then most of these have a crossover built in. Therefore you just need an amp. You could use a car amp and run it off the 12v from your PC power supply, or use a home stereo amp, or nick a small amp from some PC amplified speakers.
You'll just need a lead from sound card (3.5mm jack plug) to amp (probably 2x Phono (RCA) plugs). The speakers themselves will be easy to wire up to the amp.
I think this setup would sound pretty good (I'm thinking of using similar in my cab), if you really need a sub you'll either need an amp with sub output built in, or you will need a crossover to separate out the bass.
Hope this helps