External soundcard

Hello everyone,

Probably thread n°1000 about external soundcards, but I've got some specific questions.

Since I don't know much about hardware, I would like to know, what makes that an external soundcard enhances the quality of your music? I would also like to know what the extra possibilities are if I should get me one. And last but not least, is there a (big) difference between a soundcard running on a firewire/usb connection?

I got a budget for around 100/150 euros to buy one.

Thanks in advance..