You need to get a better understanding of the oscillator in the PIC or you'll just be beating around the bush. Since you are using a PIC18, I suggest that you download the PicMICRO 18C MCU Family Reference Manual from the MicroCHIP website and read Section 2 carefully - especially the part that talks about PLL (Phase Locked Loop).

Also, as I said in my earlier post, the maximum clock frequency (Fosc) for a PIC18 is 40MHz (64MHz for some of the newer 3.3V only parts such as the PIC18FxxKxx). This, however, is not the same thing as the crystal/resonator/external-oscillator frequency.