Do you have both installed? I think you can choose which you want to boot in a desktop manager like kdm or gdm depending on what you use now or want to use now. I think you can simply change the session to the other in the login screen.
If you start from the commandline you can use:
"startx /usr/bin/startkde" and startx /usr/bin/startgnome" the last path might be a bit different but it starts with /usr/bin/start so your search is narrowed down to only a few options by then and then you can pick out the proper one yourself too
Grub loads your distro. Then it depends on how you set the distro up. It's easiest with a desktop manager. If you like kde you can install kdm. Kdm will automatically start a graphical login then.
