if i connect something analog to a line-in on a soundcard does that mean it bypasses the source and uses the soundcard's dac or that only works for digital input?

Hi i am using headphones with my stx to listen to computer and xbox 360 audio connected to linein. but now i am considering 2.1 speakers. along with using headphones would i need an external amp for speakers can i simply connect it to my stx on rca out? would the amp improve sound? or...

right i have a hd 238 but i want to replace since it hurts after an hour or so wearing glasses   is there any full-size headphone that are small enough if u can plz list the earcup size since i want to be bigger than the hd 238 but not too big like hd 555 that is comfortable with glasses...