Even if you are miking the speaker, you'll want something in your software to fix the EQ. Even expensive mics make cabinets sound different, because, well, if you stick you ear right up on the speaker it will sound different, too. My advice is to back the mic away from the speaker a bit and to point at the cone of the speaker as opposed to the round bump in the middle. Doing this gets rid of the unnatural low and high boosts you get from having the mic so close. And crank it up as loud as you can, the hotter the signal, the lower you can turn it down in your software, and that will reduce hiss and background noise.
If you really want to get down and dirty, buy dedicated recording hardware. It will sound way better, but the same still applies, like it was said above, you want a pre-amp for the mic. By the way, many guitar amps have headphone or line out jacks already on them, even big Marshall heads. You can also run the effects loop out into the line in on your card, that will work, too, but remember that a lot of your sound is shaped by the speakers, and the signal coming out of your amp with sound way too trebley without the speakers to tone things down...
Look, you're not going to sound professional unless you dish out the $$$... But that doens't mean you can't sound pretty good and be able to make demos and do multitracking to try out parts with cheap stuff. Just depends on what your goals are.