What is the maximum serial speed one can use with the Arduino USB for communicating with a host PC?
Probably 115200.
I tried 230400, but got garbage. I think the Arduino's timing is slight off because its clock speed is not an even multiple of that speed.
Supposedly 500000 and even 1000000 work from the Arduino end, but they're non-standard, so they don't appear in most terminal programs on the computer (and may not work at all on the computer).