or
you could try putting a div with 15px padding and 650px height inside #mainContent

that's the lazy way to fix it
( Gilgamesh gazed in wonder as Frampton came alive, Sun 7 Jun 2009, 3:06, archived)

Yeah, but I found another way!
I just gave a margin to the two floating DIVs either side instead. FIXED.
( Dave Trouser ; the people's choice - 75% agree, Sun 7 Jun 2009, 3:19, archived)