where to put a little autoconfiguration script for X

Hi all!

I've made a little script that checks the VGA hardware in a computer, and makes the necessary changes to the file Xfree86 in order to get the correct modules loaded.
I'm just having troubles finding out where to put it (somewhere in rc?.d) so that it get executed after Xfree86 file being created and before the X starts.
Hope someone can help me out.

Thanks in advance