You need AMPs.
For starters, get a small 4 channel AMP that will give you ~25watts to each speaker. That would be roughly the equiv of a low to mid range head unit and they can be found pretty cheap (I think my Alpine that I briged for my rear speakers would be perfect for your case as was only about 150 (I think it's list was 200)).
Then you simply get a cable from RadioShack (or somewhere similar) that has a headphone mini jack on one end and RCA male connecters on the other. Depending on the AMP, you probably will need two female to male RCA splitters to hook up both front and rear channels.
Then when you want more power, simply put more AMPs in and use some more splitters (though there are other options then too). That is beyond the scope here though, check the Audio forum for all the info on going nuts with audio.
I currently have 3 amps (front, rear, sub) running off my mini.
As an aside, if you are using an Intel Mini, a few of us have noticed odd issues using the headphone jack directly on the Mini. I use an external M-Audio sound card (no drivers yet so all I can use is the headphone jack). The problem is that when the Mini wakes up, it no longer notices anything is plugged into the head phone jack and plays via the internal speaker. Really annoying when it is burried where it is not readily accessible.