Hi,
Goto page 183 of the 18F452 datasheet. There, you see a diagram showing how to set the conversion clock. It is from 128kHz FRC to Fosc/2 in different steps. However, you have to be careful that you provide enough acquisition and conversion times, ie, at 20MHz clock, setting Fosc/2 would not be possible, as you need atleast 13Tad time to have proper ADC functioning (check page 290). For best accuracy, I would always go for the slowest, unless I require something faster. You may want to add an RC low-pass filter at the input with 1k resistor and 0.1uF capacitor.
Hope this helps.
Tahmid.