Hey Rob,
I use this
http://pro-audio.musiciansfriend.com...ace?sku=241710
and have had wonderful luck and experiences with it. I just used an XLR cable out from my amp head into this, and this was connected via USB. It's a bit expensive, but I use it for recordings in my dorm room and it's wonderful to take to friends' houses to record with. It's also wonderful live (I use it for keyboards). It's bus powered, so you don't need an external power supply, and works great.
Otherwise, I've had good luck just getting the above-mentioned 1/4" male to 1/8" male cord and going directly from my bass to my computer. I was using a Mac, so it sounded and worked great in Garageband. It obviously won't sound as good as going through the preamp on your head, but has worked well in a pinch.
Kyle