I'm no expert, but from a quick google I found these settings for use with a 20mhz crystal from the datasheet with a 48mhz USB timing

Code:
ASM 
 __CONFIG    _CONFIG1L, _PLLDIV_5_1L & _CPUDIV_OSC1_PLL2_1L & _USBDIV_2_1L
 __CONFIG    _CONFIG1H, _FOSC_HS_1H
 __CONFIG    _CONFIG2H, _WDT_ON_2H & _WDTPS_128_2H
 __CONFIG    _CONFIG3H, _PBADEN_OFF_3H
 __CONFIG    _CONFIG4L, _LVP_OFF_4L & _ICPRT_OFF_4L & _XINST_OFF_4L
ENDASM