custom theme – DIV problem!

Hi! I am customizing a theme, and its for a blog. I want to have a header with width 805px, a menu navigation just beneath of 805px, the posts content should be the same and the footer. so the CENTER part should all be in 805px. and on the sides (both left and right) i want 160px width of sidebars, where i will put my ads. the problem now is, that everything is ok beside the left sidebar. i can put how much content i want in right sidebar without any issues, but as soon as i put too much in the left bar, the post content gets pushed down? i took a picture of example:

https://i271.photobucket.com/albums/jj124/sanianik/Skrmavbild2011-05-04kl230043.png (this is when there isnt so much text in the left sidebar)

https://i271.photobucket.com/albums/jj124/sanianik/Skrmavbild2011-05-04kl230124.png (this is when theres alot of text, and the post gets pushed down).

snippet of the code:

[CSS moderated as per the Forum Rules. Please just post a link to your site.]

thanks for the help!