Pretty much what using a digital out from your soundcard would do, except it completely eliminates the need for built in sound, you could disable it in bios and remove the driver.
In either setup you would be using an external DAC (Digital to Analogue Converter) instead of relying on the soundcard itself to do that, which is where it usually picks up interference from your other components.
Its possible it has all been bad luck though. Its easy enough to test, run a graphic intensive test/game with its sound off and listen close to the speakers/headphones with it cranked up, if you hear pure silence then your fine, if you hear background noise that rises and lowers with the graphical load then you probably want to use a external DAC.
I don't remember this happening for my old PCI cards, but I don't know if it was because something about that interface was better for soundcards, or if it was because it was the only thing using it, since Video was AGP.
seems that you have better knowledge than I do when it comes to sound peripherals, Before I build my first PC I had bought a PC from a website where they put them together for you and I noticed that specially in game the onboard sound sometimes tend to be glitch and sort of mess up with the game experience. So after I sold that PC and I ended up building my own I decided to go with whatever SC has best reviews and recommendations.
Turned out that the HT|Omega Claro + had the most and the best reviews from all brands out there, yes even put ASUS high end cards to shame, on top of it for my luck it was on sale while I was researching so it went from 200 to 100 for period of time for 24 hrs, so I bought it. I have never experienced any problems with it, here is the link of the SC
http://www.htomega.com/claroplus.html, read up the specs and tell me what you think in genereal about it, another sale for me on that card that it used only Japanese capacitors.
I will do the test that you said, but problem is it turned out that the Yamaha amplifier is on its way and the speakers are on back order... bummer... I will give them a week and see if they can get them in stock, if not ill cancel the order, get them from else where and then I can do the testing. Shame I already received the spool wire, the RCA to 3.5mm Y cable, Yami is on its way but speakers are on back order.... and their site said they had more then 10 in stock...