baloney #1
I typically use a rather common 33 MHz oscillator, as a result of which code that sets the MCU's UART to operate at 19.2 kbps, will operate at 57.6 kbps

set the code to the baudrate calculated with the frequency your oscillator runs at

The above "method" is just downrght crazy.

To set the timer to the baudrate for the actual clock frequency use this:
http://www.keil.com/c51/baudrate.asp

baloney #2
If you drive the X1 (on most 805x-types) input with a crystal oscillator rather than simply tying a crystal between the two Xn pins, and adding the load caps to GND, you'll get better results
if all you are driving is the actual uC, there is no need whatsoever for an external oscillator.

Erik