CentOS - Jun 2007 - key bindings

I am trying to bind keys on centos 5.

I want to run my own APP when I hit keypad 4 and a different APP when I 
hit keypad 5.
I was looking at xmodmap but I dont see how to execute my own command?
I was looking in the gnome keyboard shortcuts but I dont see how to 
enter a command it only lets
you change the accelerator for a given action. Not create a new action.

How can I setup keypad 4 and 5 to run app1 and app2 respectively?

THanks,

Jerry