If you just make the post column width wider, the title/tag area to the left of it has to be reduced in width (the two are constrained by an overall container div width), which means the "leave a comment" line is going to go to two lines (unless you were to reduce the font size. Same with the post title; it would have less space so it would stack more quickly to two or three lines.
Since widening the header means that you can no longer use the header uploader, it will not link back to the main site page like it does now. The new image would be put directly into the CSS instead.
There is a trick that can be used to do that that requires making a transparent image the size of the header (no background color) and saving it as a PNG image. You then put that image into a text widget in the bottom bar and link it to the site main page and then we would use positioning to move that image up and over the top of the header image.
The following will increase the overall width to 1000px from the original 800px. I've changed the width to 100% for the content so that it will always maintain the maximum width no matter what width you enter into the container/header/menu width rule. Add this to the bottom of your CSS and see what you think and then if you want to continue, we can get the header stuff straightened out for you.
#container, #header, #menu {
width: 1000px;
}
.hentry .main {
width: 100%;
}