"How can I change what Desktop environment comes up when I type in startx?" 

Some people use a different init level and they don't start directly in 
Xwindows. (probably because they like to use the CLI a lot more! ;)

But this is fairly easy. all you have to do is edit the .xinitrc file in 
your home directory to start whatever desktop you want. 
To do this start up your favorite text editor (eg: vim, vi, pico, emacs)
and edit (or create) your .xinitrc file in your home directory. Like this: 

Code:

vim $HOME/.xinitrc 

then add the "exec" followed by the desktop of your choice to the first line. 
eg: 

Code:

exec startkde 

Here are the commands I know of: 

exec startkde                    ----> #for kde 
exec gnome-session               ----> #for gnome 
exec wmaker                      ----> #for windowmaker 
exec enlightenment               ----> #for enlightenment 

and thats all there is to it. Save and exit.