Yeah, going from the RCA of a powerful mixer component to a mic-input of a soundcard is not the best thing. You can add bunchies of improvement there. If you can update your soundcard. Try looking for something that is intended for the purpose of recording. I wouldn't ditch your mixer, you just need something that will give you a better transfer from mixer to comp. As doc already stated, the M-box will help alot.

kick drum

100 hz is where the punch is. Also a boost around 3 k and 8 k will give a "smack" sound". I usually aim the mic right at the point where the hammer hits the head. You may also want to use a noise gate. You just want to capture the smack and that's about it. So your bass drums sounds like "boom" not "booooommmmmm". None of the latter ringing out sound. Short and sweet. Compress the signal to your liking. You also don't want to use any reverb, echo or chorus on the bass drum. You want it right in your face, knocking you down right. Then don't make it sound like it's at the other end of a train tunnel.