Bigass plug? Im thinking XLR here, but its just an assumption. At any rate, switching out soundcard may or may not be important, depending on what you have. I went from a Creative Soundblaster card from last millennium to a half-pro M-Audio 2496 or something studio soundcard. Be prepared for some major bitching around with settings if you're a newbie at it(like i was). Thing is that you will notice a GREAT improvement in sound quality.
As for USB im not sure what you mean, do you mean an XLR(or whatever that bigassplug thing is) To USB cable? I dont think those exist, if thats what you mean. i think you'll have to go for an XLR to Jack, and then jack to minijack, if you have a normal soundcard. If you get a studio soundcard like me, you can get a XLR to Phono cable instead, it will save some of the signal quality.
Oh and a tip, when recording, put the amp and guitar as far away from the comp you get, it makes a hell of a noise.