About the CPU cycles: Audio hit is minimal, with your logic of consuming CPU cycles, then the other several programs running in background should make the game unplayable since they consume more CPU resources than audio. I run a lot of programs, sometimes even another game or folding, and i never notice any performance variation what so ever.
About the soundcard: sound is being processed in the soundcard, that´s the whole point. But it was several years ago when a soundcard helped boost performance, today the only difference is sound quality. About the ram in your soundcard, it was a Creative gimmick, benchmarks between with SB Titaniums (one with the ram and the other without, don´t remember the exact models though) showed that in a PC with Core 2 Duo or Quads the performance difference was of 1fps well within the error margin.
With all due respect, i still think there is something messed up in your PC, maybe drivers, who knows. As i said, games running faster is one thing, but looking better, i just don´t see how audio have anything to do with that.