I just installed Arch Linux (Latest stable ISO from website) and I need to know whether or not I can just go straight to GNOME/KDE/LXDE, etc. without installing X11 first.
X11 and desktop environments play different roles.
X11 combines the following main features, from the lower level upwards:
X11 has no viable alternate. It's part of every Unix system's GUI layer.
On top of X11, you need a window manager. The window manager handles things like decorations on windows (borders, title bars, etc.), window manipulations (moving and resizing windows, icons, etc.), focus strategies (e.g. focusing a window when you click on it), etc. X11 contains a built-in window manager, but it's extremely crude.
Some, but not all, window managers are part of a desktop environment. A desktop environment provides additional features like ways to launch programs (e.g. menus), applets (clock, workspace switcher, configuration interface, …), and so on. Desktop environment often come with a suite of applications that have the same look and feel but can be used independently as well.
Gnome, KDE and LXDE are all desktop environments. They come with a window manager (Gnome gives you a choice, KDE and LXDE have their own).
You need X11 if you want to use those environments on your local machine (most of the time you want this).