preferred applications in gnome/nautilus

[Joep L. Blom]

I have a small annoying problem because I can't find (or have forgotten) 
where the applications, visible in nautilus, can be changed
(when right-clicking on a filename a menu pops up with a list of 
applications).
I've looked on all items under System-> Preferences and System-> 
Administration but nothing is there and the help information is not very 
helpful.
I want e.g. to start OO-files to read with OO 3.01 but nautilus finds 
only OO 2.4, and it will stubornly try to start khexedit but I want to 
use Ghex (I use Gnome from the start).
It must be something simple but I can't find it and hope somebody has
more knowledge and can help me out.
My system: AMD64 OS: Hardy.
Thanks in advance,
Joep