I used the same logic and had interesting results. Had a dedicated Audigy2 PCI sound card installed, had nothing but trouble with sound drop outs and crackling...thought it was MWLL but finally took it out and went to using the onboard sound on my motherboard. You would think it would perform worse. It got better though. Not completely better but better. Then later on I bought a headset for using Teamspeak/Ventrillo and gaming with. I messed up and realized after the fact I got a USB headset which of course because it is USB, has its own sound DSP chip inside of itself so it doesnt even use the onboard sound processor. I figured it would be completely worthless. Surprise! I have almost no audio issues now. Now, I know there are some progressive fixes with version releases that addressed sound issues superimposed over this anecdotal evidence. I have been wanting to re-install my Audigy2 sound card to see if it is still not working properly....
I think it is basically pot luck on a few things.
A) Sound doesn't generally effect the CPU much at all, so it is more about the amount of channels you can have, and the sound quality it self
B) How good your onboard sound is
C) Which sound card you buy, some don't work at all, some do seem to work
D) How lucky you are
The problem comes down to D) where in the end, you could just be throwing money away for nothing.
So basically if you got money to spare, and it's annoying you that badly, I would check the fourms on who's managed to get it to work with which soundcard, and buy one. Otherwise I wouldn't waste your money.
It's very rare I get any sound problems and I only use onboard sound. I would never ever buy a soundcard for this game since it just doesn't need it in my experiance and would be a waste of money.
Experiences will vary, but that was mine.
This.