Ex:9600 baud FSK signal centered at 440MHz still needs something capable of capturing around 440MHz, but this has nothing to do with baud rate. A 500MHz scope would do well for acquiring the signal.

Practically radios often divide the problem into two parts, first use a mixer to shift the signal down and filter out the parts you don't want, then decode the FSK which remains at a much lower frequency.

This is the governing limit: https://en.m.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theo... and it considers things like SNR as well.