There are lots of USB pre-amp devices around.If you only require one input I use a Line6 Toneport GX,
http://line6.com/podstudiogx/
Comes with great amp modelling software as well as a really good in-built sound card.Very portable and no latency as you can monitor all the sounds direct in headphones or speakers whilst you record into Reaper.They have larger models.It's very forgiving as well as I now actually run an ART tube pre-amp into the GX and then into Reaper