Might be a bit of confusion here.

The 18F1320 for example has an internal OSC that can run up to 8MHz.
If you use an external OSC then you can set HSPLL in the configs and then the chip will run four times the external speed.

In the data sheet look at section 2.0 Oscillator Configurations. Then to see all of the config options goto your MPASM directory and find the *.inc file for the chip. Near the EOF the possible configs are listed.

Then look at this thread to get a handle on coding the configs.
http://www.picbasic.co.uk/forum/showthread.php?t=543