You will need a DI box or a preamp that doubles as one. But, if it's electric guitar it probably won't sound great without going through an amp, and if it's acoustic it won't sound great because most acoustic guitar pickups suck.
use a di box. i've tried running straight into the sound card, the line input of my mixer into the sound card, and a line out of my amp into the sound card. the di box has the least amount of noise and sounds best to me.
