I have a lot of experience with guitar to pc and I agree with Neal that coming out of the pc or mac back into a guitar amp is not the best thing to do. There are a lot of software solutions out there now that negate the need for an amp in a home set up. I admit that I've spent a lot of money on software...I own Sonar producer 8.5...Ik Multimedia Amplitube 3...Also Jimi Hendrix...Fender and Metal...and also Ik Multimedia Ampeg for bass... but to be honest most of the freeware is not up to much...but also the very best plug ins are not that expensive compared to buying a couple of top notch amps...!!!

As Neal said...the Behringer USB is pretty cheep and easy to use as an interface...

If you have any questions about specific software then I would be happy to help...I own a copy of just about everything...have no money left but love my set up...!!!