[CentOS] Memory Usage in Top and System Monitor

Can someone help clear up some confusion in reading memory usage in Top and System monitor. Here is a picture of both.
http://s1176.photobucket.com/albums/x327/ionosphere2011/

Why does "System Monitor" show 1.7gb free out of 8gb.  While Top shows all 8gb being used?
If 1.7gb is free then it should not be using swap space so I assume "System Monitor" is reporting it incorrectly?

Thanks

-- Mike

 		 	   		  
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