The mic in of a cheap soundcard goes into a preamp but it's pretty pitiful and you won't get a very pleasant sound by plugging a guitar signal into it. It's so bad it makes you not feel like playing.
Some kind of external preamp is the only way to go, really. There are basically two avenues:
(1) A guitar preamp/processor of some sort. This includes most modern amps, which often have a Line Out from the preamp section. Then there are boxes like those Digitech and ART rack-mounted guitar effects boxes, and floor pedals like the newer ART and Digitech and Korg and Zoom. These typically have line-level outs and you can get a pretty decent sound from them, certainly when compared to the soundcard's mic pre. Finally, these days there are many devices available that do some sort of sophisticated software "modeling" of the sound of a great amp miked well (the fabled POD,
the Johnson J-Station,
the Yamaha DG-Stomp, and others). Some people hate them, some people love them. The advantages to this approach are that you can get a very good to excellent sound without all the effort needed to fiddle with mic placement in front of an amp, and without the sound pressure levels you typically need to achieve to get the amp voicing in its happy zone -- a great thing when you live in a small place where loud sound can mean troubles with the neighbors or the family. Prices range from maybe around $100 to over $1000, with many great choices in the sub-$500 range.
(2) The traditional mic-pointed-at-the-speaker. This requires a decent amp, a decent mic, and some kind of preamp for the microphone. Assuming that you already have an amp you are happy with, the general wisdom is that a Shure SM-57 pointed at the speaker cone (recommended angles and distances vary somewhat), and preamped through a mixer to bring the signal to line level, is the way to go. The advantages are that you can truly capture something like the "real sound" of a great amp -- ain't no way no weenie software modeler can ever match the ultimate zenith of sonic perfection offered by this approach, dude
. Prices -- an SM-57 goes for about $80, and you can get a small ART Tube MP preamp for about $100, or a small Behringer mixer with a few mic preamps for between $100 and $200 depending on the model. Most people here will swear by Mackie mixers, which are a bit more expensive. There are also a lot of preamps available, at all price points, with many very good ones in the under $500 range. Oh, and don't forget a decent mic stand to hold that puppy in the right spot once you find it...