I would have said the Auzentech X-Meridian 7.1, but since thats discontinued... It would be between the Auzentech Prelude and the Asus Xonar. Are you using analog or digital?
xonar d2 pips it for audio. though spending £100 on a soundcard doesnt make much sence when your using pc speakers. you wouldnt be getting the best from it
I have found them to be ok, not messed around with them. Just changed the in/outputthe vista drivers are pretty buggy actuallybut i cant grumble with the sound quality