What's the difference between a PCIE sound card and a built-in motherboard sound? Is the difference as drastic as "onboard video" vs. "graphics card"? Do you really hear any difference...besides the fact that some can do 7.1 for more speakers, etc?
Do different brands/prices of sound cards really matter?
There are too many variables to answer with a simple yes or no (for either option). Things that must be asked first are (and please note, everything here can be argued, so these are not absolute examples):
1. Speaker system: How good are the speakers? No matter what, the sound will always be "reproduced" by the speakers, so that's a critical element right there. You can have the best of the best soundcard, but the sound you will hear will suck coming from that stereo speaker set you stole from work (we all know what I am talking about).
2. Which soundcard vs which onboard audio? There are really and truly terrible soundcards out there, and yes, great quality onboard audio chips exist also.
3. Budget. Self-explanatory
4. Use. What are you using it for?
This issue is not quite the same as the onboard video vs dedicated video card, because the differences between those can be quantified (you can measure FPS, as an example) but how can you measure what sounds better? Ask 3 people to listen to something in a room (for quality) and all 3 will have different opinions.
As for the price and brand - yes, they do matter. You will always get better tech with higher models, and higher models always cost more. Brand does matter for driver support and "reliability", although there isn't a flood of options. I can think of 4 brands only, and they are all well respected brands. Stay away from generic brands (aka Best Buy brand, etc. - and yes they exist).