Correct place to put keymap-modifying script?

[Danny Colligan]

I want to switch my keymaps such that the Esc key triggers the
Caps_Lock function and the Caps_Lock triggers the Esc function,
essentially switching the functionality of the keys.  To enable this
functionality, I need to run a command like xmodmap
.switch_esc_and_capslock where .switch_esc_and_capslock contains the
necessary configurations.  Of course, it would be nice to have this
command run automatically by placing it in some kind of startup file.
Which startup file would be the correct place to put this command if I
wanted it to run across all of X and only when I log in?  I'm using
the standard X/GNOME Ubuntu configuration.

Thanks in advance,

Danny