"For the money" it's tough to beat a $3 adaptor from Radio Shack.
I did that for years, dude, before I bought my current adaptor. That and a mic and a XLR-1/4" cable are all you need.
If you can get away with it, one of the nice bits about the line in is that you can usually monitor your signal through your soundcard, but the mic in is generally the hotter of the signals, so you might not get a robust enough sound to work with.
EDIT - also, my usual speech - micing an amp and getting a tone on disk is an absolute art form, and if what you're trying to do here is to learn how to use a microphone musically to capture a source sound, as good as anytthing from Line6 may sound in the short term, you're not going to learn how to mic an amp, and it's really one of those artforms that is very difficult to learn in any other manner than just doing it a LOT. So, Gearbox will do the trick in the short term, but it's ultimately going to limit you in the long run because it puts you way behind on the mic selection/positioning learning curve.
Besides (and I never thought I'd ever get to crack on someone else for this around here
) I can't recall having heard a whole lot of recordings from you, Jason, which makes me wonder if you've really gotten your money's worth from that thing.