I was also looking into that as well. I was wondering if anyone could tell me which would be cheaper and which would have the better quality (if noticeable) between recording via the Toneport and software it comes with, or by just buying a single-input valve preamp i can get for £60 from a nearby shop, based on the fact that i don't particularly wanna use the guitar inputs, i wanna mic my amp and will obviously use the mic input for vocals etc as well. Any advice would be welcome.
Also i am recording into my laptop, so it doesn't have all the inputs of a standard pc, has microphone input and USB etc, but no firewire so it rules out most DI boxes.

And in answer to the threadstarter, the soundcard will only make a difference to the sound when you play it back on your computer, so you don't need a new soundcard, no.