(Another question I have is how reliable serial comm with the 8051 is. Can I assume that if the 8051 sends a byte, the PC will receive the right data? )

Use an Oscillator to test communication between mc and P.C
For example use baud rate 1200bps 1stop bit and no parity send from mc to p.C
10101010
mov sbuf,#10101010B
connect oscillator in p3.1 and see the waveform the pulse width must be approximately 840us (1/840=1200).
Do the opposite from p.c to mC
If you don't use the same baud rate it wouldn't work properly