i think sound quality is going to take a hit with that. Since you are going mic'ing a amp, to a "sound board" and plugging into your sound card.
So you are relying on your sound card to carry the sound.
I would suggest this: http://www.zzounds.com/item--PRSFSMOBILE
Its a firebox, it is connected via FIREWIRE its pricey. but the sound quality is that of a studio.
I used an older model for a long time, and the sound quality was seriously CD.
and USB units are nice, but slow, and sound quality will take the hit. Use a firewire for the best.