Home studio recording help?

Alright, I'm basically starting out from scratch with some simple PC recording, but I have some experience. I have a Behringer Xenyx 802 mixer, but using an RCA cable from the mixer to the mic input on my laptop sounded awful, because the sound card in my laptop is bad. Would a Behringer UCA202 sound card via USB fix that problem? Also what cables would I need to hook the UCA202 to my mixer? As for recording electric guitar/bass, would a DI box be a good idea? If I plug my guitar into the 1/4" input of the DI box, and the 1/4" output from the DI box to my guitar amp, and then the XLR output into the XLR input of my mixer, would that work? Also would using a DI box record the sound directly from the guitar pickups, or the sounds/effects from my amp? Or in a completely different direction, would I be better off just recording my cab with a Shure SM57? I have heard that a Shure wont deliver as good quality sound with bass as with guitar. Please give detailed answers, as I'm pretty new to this stuff. Thank You