without a head unit i believe you are going to need an amp. the simplest way would be to get a 2 channel input with 4 channel output, with an internal crossover. that way you can input right and left, and set the amp to output the highs to the front (typically smaller speakers) and the lows to the back (typically larger speakers)
for a sound card, you probably want to look for a card with rca outputs, since that is the input for the amps. i personally would recommend a sound blaster awe 64 gold sound card. it is an ISA card, and comes with right/left rca outputs.
now this is mostly just an idea that i am tossing your way. i havent personally set up anything like this, but i have seen it done somewhere on the web. there may even be some folks here who have do similar setups.
oh, you might also want to look for a radio tuner card for your computer, in case you want to tune into sports, news, traffic, weather, etc. basially anything that isnt an mp3.
you will probably also want to look into a touch screen, since all of the functions of the radio are going to be controlled by your computer.