I'm using the same internal soundcard and don't have much problems with my sound quality. In my setup though I've got two outs on my mixer, the main output goes to my hifi and a seperate booth output goes into the line in on my PC. Checking your levels is well worth it, I don't trust my mixers vu meter and I always calibrate it when recording a mix to whatever the levels in Soundforge tell me are correct.

The key thing to remember when you record is, if you redline it you are ****ing your mix permenantly, there is no way to recover lost information if you distort it. If you record it too quiet and err on the side of caution then your mix comes out a bit quiet but you can pump the volume up in post production to get it back to the right levels so it's always worth being safer than sorry.