@Robin2, I believe you that 1000000 will cause zero error at an 328p, but that was not what I asked - so thanks for your input, but sorry, your posts don't help me - you'd better move along.
@Grumpy_Mike:
if you are not willing or able to tell me the max clock speed, so let me again rephrase my question:
which subset of the following UART clock freqencies
230400, 460800, 500000, 576000, 921600, 1000000, 1152000, 1500000, 2000000, 2500000, 3000000, 3500000 and 4000000
will work by Serial.begin(x) with either one of the following cpu boards:
16MHz Nano
16MHz Uno
16MHz Mega2560
Zero (M0, 48 MHz cpu)
Due (M3, 84 MHz cpu)
and esp8266 (80 MHz cpu)