hmm, i guess first off, really just about any current add on sound card would be better than integrated sound, integrated sound can be decent, but seeing as you dont seem to have any complaints yourself, but almost rest assured, it wont sound nearly as good as a high quality add on sound card (though that is entirely subjective, from person to person)... ...for simply listening to music, over just a 2 channel analog connection (2 channel analog stereo listening is usually best for music)... ...i would say what to go for would be a an add on sound card with a low THD level, low signal to noise ratio, high quality D/A converters, high quality operational amplifiers(?)... im sure someone else could be more specific in what you should look for
surround is abit of a different story (though you would still want high quality sound nonetheless)... for surround, which is more tailored to movies and gaming, but still excellent for music... ...the first thing you would need though, and the most important component, are the additional speakers, preferrably with almost identical matching components, construction, specifics and such, so they dont sound too different from one another (one speaker sounds brighter and shallow, and another sounds more dull and bass heavy, ones loud, ones quiet, etc)... all in one 5.1 speaker sets are good places to start... so you could right away have a 7.1 set up, instead of purchasing speakers individually, if you add your current speakers as front channels or so (you might be able to go to a local retailer and 'test out' a few speaker sets they have set up, to see if you like how they sound or not, and if theyre within your budget)... ...also to check how many channels your amplifier supports... and then purchase the amount of additional speakers based on that...
also: as far as quality balancing between your amplifier and speakers, in regards to overall bright, dull, harsh, soft, etc sounds from your speakers... and i dont know if you could find this out without searching online, but lets say for instance, that your receiver naturally outputs more of a 'soft' or 'dull' sound... you would then want speakers that can counterbalance that, for them to be naturally more 'harsh' or 'bright' sounding, so your overall sound quality ends up being more balanced...
i know i keep recommending this, and other people will say an x-fi sound card for gaming... but for surround, ill opt to say any card that can re encode the sound to DD or DTS in realtime... THAT, i think will make for one of the biggest changes to your overall surround sound...
...heres a personal review i wrote earlier today, as a request from another member on here (its towards the bottom of the page):
http://forumz.tomshardware.com/hardware/FL-FR-channels-ftopict190526.html
just like Michaelahess said though, he beat me to it, lol... if only to completely retract what i said, and to agree really... ...if you dont have any problems or inconveniences with what you already have, and are happy... then theres really no reason to change anything.