On Sat, Aug 2, 2014 at 9:55 AM, Ken Restivo <ken@restivo.org> wrote:
as soon as a
1Ghz board with maybe 1GB RAM or 2GB RAM and well-designed dual-USB
connectivity comes down into the under-$100 range, there's the synth
I was hoping to build in 2007

Not sure about USB performance, but you get a sub $100 board with a 1.7GHz quad-core processor and 2GB RAM already. See here:

http://hardkernel.com/main/products/prdt_info.php?g_code=G138745696275

Their Pi compatible board is also interesting. For about $60 you can get something smaller than the original Pi with a screen, battery regulator/charger, extra USB ports, and additional ADC.

http://hardkernel.com/main/products/prdt_info.php?g_code=G140610189490
http://hardkernel.com/main/products/prdt_info.php?g_code=G140609436593