depents on the mC for example you can use up to 24MHz crystal for AT89S8252 .
If you want you can have a variable baud rate for serial communications..use mC in mode 1 (rs232 style 1 start bit 8data bits and 1stop bit)and use timer1 as baud rate genarator the formula is:
TH1={256-[(k*f)/(384*b.r)]}
k=1 if pcon.7=0
k=2 if pcon.7=1

pcon.7==SMOD
b.r=baud rate
f= os frequency

example:

f=11.0592MHz
k=1
TH1=FD(=253d)

==> b.r=9600bps