1. You can add external sound card/additional outputs in several ways. If you want to get a full 5.1 sound, the easiest way is to get Griffin FireWave or similar interface. If you are fine with 4 channels, you can get either Edirol UA-1EX or Griffin iMic and then create a virtual composite device in Audio MIDI setup (it will have 2 channes from one device + 2 from the other).
2. Unless your speakers are something really exotic, any amps should work with them. There are two basic rules: the power rating of your speakers should be more than power rating (RMS) of your amp and the impedance of the speakers must be within the range the amp can handle. If you ignore the first rule and try full volume, you may destroy your speakers, if you ignore the second one, you may fry your amp (although most of them will have some kind of protection).
3. At the moment, I am using the built-in audio output of the Mac mini + an old Sony head unit with Aux input in my Volvo and an Edirol UA-1EX + Sony head unit + two Audison SRx amplifiers (4 ch for speakers and 2 ch bridged for sub). I plan to play a bit with HQCT eA USB tuner within a month or so.