I use the Sound Devices USB Pre in my studio & on the road.
Link here:
http://www.sounddevices.com/products/usbpremaster.htm
the biggest plus is it has an awesome mic pre-amp…really high quality. It will also take ¼” inputs, digital inputs, etc. It’s very well built, and it costs $550 or so. I just bought one on ebay for $350 so I can keep it in my road bag, so you can sometimes find them used, but not often. I’ve been looking for six months.
Another one that might make sense would be this one:
http://www.m-audio.com/products/en_us/FastTrackPro-main.html
$225 or so. One benefit would be the ‘insert channel’ so you could run a tiny compressor or something similar to make sure the sound was already compressed a little on the way in to the computer. If you want a really good compressor that is tiny and cheap, this one is the best there is for under $500:
http://www.fmraudio.com/RNC1773.HTM
looks like junk, sounds really good. It’s $199.
If you wanted to go as cheap as possible on the sound card, the one that would still sound 90% as good would be this one (it doesn’t have the insert patch for an outboard compressor):
http://www.m-audio.com/products/en_us/MobilePreUSB-main.html
it’s about $150.