Hi Steven,
1. Mapping into 12-bit greyscale is a nonsence. You need to perform a log compression into 8-bit greyscale.
2. Generaly, your idea is OK. In order to make your design simple and flexible, I suggest to do it like this:
Probe<=>TX/RX=>AFE5808A=>FPGA=>USB(FT2232H)=>PC+software
Mariusz
