You just need to have enough partitions, one for each install (plus, as mentioned above, probably a good idea to have your data outside of those, so you can access it from either installation). At install time, leave one partition empty and then let the other installer install to that. They should be perfectly able to coexist peacefully, in my experience.

Can't say much about KDE, I've been using Gnome and never saw any reason to try K. I basically just use the GUI to launch my music player and browser anyway, and everything else happens in Emacs or the terminal.

They seem to have a wider spectrum of software written specifically for KDE but most of it will run in Gnome just fine as well. From a programmer's point of view I understand Qt (the KDE toolkit) is pretty much best in class, but I haven't done any GUI programming myself, so can't really comment on that. My prejudist opinion is that KDE seems more oriented towards eye candy, but my KDE friend says Gnome is impossible to use without a mouse, whereas he routinely uses KDE from the keyboard only. Anyway, both of them have a fair share of programs which quite horrible usability -- I guess a lot of developers assume it will Just Work as long as they have an all-dancing, all-singing animated color-flashing GUI. (Ahem. Gets off soapbox. Pardon me.)