I've got a M-Audio Black Box. It cost me about £250, and I plug my guitar into that, then into USB. It's got decent amp-emulation and sounds amazing. The only drawback is the software that comes with it (Ableton). You only get a demo version and a full one would cost you the same difference again.

There is always the Line 6 Rifftracker. Does a pretty similar job, costs less and has good reviews. Don't know if it's as good though cos I haven't compared them.