I have an internal Audigy-something 7.1 and an external M-audio Fast track USB sound card where I can connect music instruments.
I use mainly internal mostly. In terms of sound quality during playback I can't tell the difference between the two. Gaming, music or whatever I do.
When it comes to onboard, I can. And for some reason I just can't get the volume to go high enough for my liking.
Onboard used to be heavier on CPU years ago but nowadays that don't matter. CPUs are just so fast. Same deal with LAN port.
I think the CPU utilization of Onboard sound is around 2-3%.
The reason to get a music soundcard, be it external or internal, is latency.
Normal soundcards, like Soundblaster and Audigy have lots of latency during recording, like 200 ms. That is unacceptable during music recording. You play a note but you hear it way later. It it very disruptive. Music is all about tempo and when the instrument I play actually plays 1/5 seconds later, that throws me off totally.
With a music production card that latency is like 8-20 ms. Can't even notice the latency.
I think it has been proved scientifically that we can't hear latencies around 10ms, ever.
I can't understand people who use onboard sound. Makes no sense to me. The question I ask myself is: Are these people deaf?
I can clearly hear how crappy the sound is.