> Now that I've got KDE going, I'm thinking about Gnome. 
> Tell me please, is there any reason why I aught not to 
> also install Gnome? I expect that I could only run one at 
> a time. But is there any reason why I could not keep KDE 
> installed for me as "user A" and then also set myself up 
> as "user B" with Gnome for X when I'm user-B?


It is quite reasonable to install both if you wish to use both (and have 
the diskspace ;o).  You can switch between them by adjusting the 
.xinitrc or .xsession files in the home directory for each user.

Mike.

P.S. This is a fine list for this question in my opinion.