Mike the microphone in between is by far the biggest problem and the recorder step seems redundant causing additional fidelity loss. If you use the headphones out mini plug on the cassette and go directly into the “line in” of the sound card it will lose very little and retain stereo.
http://www.radioshack.com/product/index.jsp?productId=3812787 Amazon.com also has this stuff. Being a laptop it may not have line in and the only option could be “microphone in”. If using a microphone port make sure the signal boost is off.
If it goes through the amp it will lose stereo although you can still make up the two channels in Audacity. From cassette, to amp, line out from amp to sound card line in. They have 1/4” (From amp) to the min plug (To sound card line in) adapters
http://www.radioshack.com/product/index.jsp?productId=2102952 .
If you get rid of the microphone and recorder in the chain and the sound quality will improve a huge amount. It will be split mono (just record to a stereo track), but as stereo as it can be coming from a mono amp.
Audacity now supports VSTs so if you have a particular effect on the amp you want across the whole mix, you may be able to add something very close afterwards:
http://wiki.audacityteam.org/index.php?title=VST_Plug-ins