If you'd like to make a menu item open in a lightbox, you can use the Lightbox2 module in tandem with the Menu Attributes module module to achieve a variety of effects.
Download and enable the Menu Attributes module.
This will let you add a rel attribute to any menu item. Add "lightframe" or one of the following rel values (as appropriate):
See here to learn how to put the node content only into a lightbox (stripping out the sidebars, etc).

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