If you install Launcher Pro (advice: buy Plus version), you will have essentially an operating system to replace your home screens. It is much more customizable and the whole of the X's operations works faster.
DroidXcon is right that you can activate it or deactivate it (returning to the stock home screens) using the free download app Home Switcher. Also, you can designate (with Home Switcher) which launcher is the default.
HOWEVER, you should clear your home screens of everything before you activate the LP+. Otherwise, you will have an X home screen working in the background behind the LP+ screens. Battery drain; potential widget conflicts, and such.
LP+ only controls the home screens, dock and app launcher. That's it.