External Monitor question

I installed Ubuntu 7.04 on my Dell Laptop 8600.  My laptip monitor and external work.  My only question is both screens shows the same things.  WINXP used to show both monitors but both worked togehter to give me one desktop.  The left (laptop) monitor showed and then when I moved the mouse over to the right the mouse would move to the right (Old HP Pavlion External) monitor which gave me more desktop space.  Now on Ubuntu 7.04 have to two monitors which show exactly the same things (which is useless).Any hints?  Thanks in advance.