If you want to have a background across more than one column have you tried putting both columns inside a containing div, and then using a background image on that containing div?
<div class="column1">left hand nav - this one has a float:left, width 200px</div>
<div class="column2">main page - this one also has float:left; width:700px</div>
Now if you make the width of the container 900 and create an image that is repeated vertically, it will ensure that your gradient background is consistent across all columns...
Is that the kinda thing you were trying for?