You can use any x-tal frequency available in the range supported by your controller, but have in mind, that not all baud rates are possible. You have to calculate reload value for baud-rate timer based upon x-tal frequency. +- 1% deviation of exact value is acceptable.
I suggest you use 8032 timer T2 as a baud rate timer generator. There you have more options than you have when using timer T1. At 12 MHz x-tal, you can get max. 28800 baud (reload value is -13), but if you are using timer T1, max. baud rate is 2400 baud.