Here are my experiences with computer audio.
I started with dedicated sound since my first PC (Athlon 800 Thunderbird/ge-force 256) and have worked my way through the sound blaster series cards through the years until driver support dropped.
I am not really a headphone man, I tend to only use headphones when it would be inappropriate to have my speakers running. Receivers, Amps and semi decent speaker/subs are what I have always driven, up until my latest set up.
My computer leads into an electronic crossover and then into two amps, one for treble and one for bass/mid. These drive a pair of speakers I designed specifically to be bi-amped. These tend to be very transparent speakers with an clear uncluttered sound, but a bit of warmth would not go amiss.
I originally had an x-fi extreme audio card which sounded decent, but was nothing special.
I modded it by upgrading the op amps and capacitors, this made a huge difference in sound quality. The sound opened up - everything was so much more clear and detailed, music seemed far less muffled than before.
I then moved do an external DAC (grubdac) - a difference of night and day from the sound card! Everything just sounded so precise, all the detail was there. I found the greatest benefit was lower down, mid bass was clear and detailed. At the bottom end, things had lost the boomy sound the x-fi threw out, the sound was clear and extended.
I received a pair of HD515's last Christmas, with my DAC driving my amps I ended running them off my computers on-board sound.
With SupremeFX X-Fi sound built in I figured this would be OK, I just accepted that my headphones had nothing on my full speaker set up and left things at that for the last 10 months.
This last month and a half have seen me go past a surplus electronics store daily on the way to work. After having a look around I found a rack of old tubes they were still trying to clear out. I picked up a couple of pentodes that I knew I could make would work with the millet starving student headphone amp.
I have been working on the amp, a bit at a time over the last couple of weeks and got it to a functioning state last weekend. I hooked it up to my ipod and some old headphones to test - sounded OK. Next up were my 515's and GrubDAC - I could not believe how good my headphones sounded, far better than any headphone system I have ever heard.
This still does not sound as good as my speaker/crossover/amp setup, but the difference between DAC/headphone amp and onboard sound is quite astounding.
I would recommend moving off on-board sound into a DAC or high end sound card to anyone.
Remember that sound is analogue and a digital out is only as good on the DAC on the other side - a cheap receiver will have a cheap DAC.
I have heard receivers that sound worse, fed with digital than with an analogue source from a media PC.