for ET there are two important files that store your information.
silent.dat and etkey
you should be able to find them in your etmain and silent folder. Definitively save them.
other things to consider:
- if you have any scripts like autoexec etconfig etc, you might want to save them
- you can copy all the maps over, they are in the etmain folder (however, if you dont do that, it is not a big problem, your client will download them)