Most visual elements in a graph can be reordered using the Customize option from the right-click menu:
7176_ContourGridCustomize.png
From the Customize dialog, select the Grid Lines object and use the down-arrow button to position it below the Contour object (which sounds counterintuitive - this list is the order in which objects are drawn):
7177_ContourGridReorder.png
Result:
7181_ContourGridOver.png
Hope that helps!