if you get a 2.1 system you probably cannot add anymore speakers later on.
yes you can connect headphones to your soundcard's headphone port.
personally, i prefer nice 2.1 over cheap 5.1. I'm big on sound quality and am constantly listening to music..... more so than I play games with 5.1 sound
is your home theater amp stereo? 2.1? 5.1? 7.1? If it's stereo or 2.1, you can run a 1/8" to RCA cable from the soundcard line-out to the stereo RCA inputs on the back of your amp. If it's 5.1, you can do the same thing, but you won't have true 5.1 sound.
If your amp has optical input, you can hook it to your soundcard with that provided your soundcard has optical output. this will give you true 5.1 sound (when supported)