categories page how do you extend the margins?

[alex007f]

I have the header width 766px ,the left column is width 207px,this is OK for me ,well, I need the body to be width 459 px. ;Body is -3 columns of products,the images have no width setteled in admin becouse i don't want to distortion the image,I keep only the high of the image.The margins of my website are black .The question is that even if i don't insert any product i want the body which is white color to be 459 px ,in order to fit with the header.

I apreaciate a lot any help for this problem thatt i have,

Thank you very much in advance.