1. hifi or home theater speakers do generally sound better than computer speakers. some bookshelf sized designs can hit bass notes but do not extend very low due to no subwoofer correct (you can add one in the future). *see below for more
2. no, but i do know the av40's are also very popular and come recommended. i have heard that they have a bit of a quality control issue though. even the mackie cr series isnt bad. you arent just limited to the few choices i listed above. but i'm trying to leave you room in case you wanted to upgrade your internal dac. if you did not care about that, you could of course spend more ** see below for more
3. fair to middling. generally high end onboard (on expensive motherboards) is generally equal to $30 soundcards like the xonar dg in quality, sometimes just a bit better. any decent card is going to be better. with that said, unless you are noticing issue like static, crackling, buzzing, distortion, etc onboard is often "good enough" and upgrading to better speakers will result in a better perceived "upgrade" than just changing out the soundcard.
* i owned the promedia 2.1 for a few days, we have the z130, i've heard the z323 and z623 in the store. i personally thought the promedia had promise but the subwoofer is sized far too large for the speakers and overpowers them. i was so impressed that i ended up buying home theater speakers from klipsch instead and have been using them since. logitech speakers tend towards average sound quality and big boomy but not rumbly or tight bass. good value but not high end sound.
** as long as your onboard audio doesnt show signs of problems it should be safe to just spend your budget on speakers or speakers and an amplifier. if you have issues, you might want to get a better dac or soundcard.
as far as that $2 codec...
there is a whole lot of "ifs" and "assumptions" to be made when saying that i'll briefly list a few:
- the listeners ears. not everyone can perceive audio in the same way, tone levels or complexity. what sounds good to me may sound terrible to you. what they said about sound file quality also relates to this.
- build quality. some onboard has issues like EMI, distortion or other issues to worry about or poor quality jacks
- poor amplification. some onboard has weak amplifiers which are completely incapable of powering high demand headphones or high %thd which really shows on lower quality songs
- headphones used. some headphones point out audio flaws like salt on mirror while others ignore subtleties so you might not hear it.
- lack of features. soundcards are specialized and onboard even more limited. while they work for the vast majority of people there may be features that are required by some people not supported by onboard.
basically that whole article only serves to say one thing which i mentioned above.. for most people onboard audio is sufficient unless you are having onboard audio issues or you find it lacking in quality. there is a reason why high end equipment is available, but not everyone needs it.