These are just my opinions on the layout which I'm sure other people will disagree with because everyone has their own 'eye' for design. I would get rid of the white and the border around the 'content container' and just let that part of the page be the same as the rest of the background color.
Image replacement allows you to use a background-image in certain tags (e.g. <a>, <h1>, etc) while still making the HTML look like <h1>My Header</h1> - this is good for three main reasons (as far as I see it, maybe there are more)
(1) Visual Appeal (allows you to use an image in case you want to use a specific font that isn't on your visitors computers)
(2) if the visitor has CSS turned off, then the text will be visible
(3) When search engines look at your page and scan through looking for keywords in links, headers, paragraphs, etc this: <h1><img src="myimage.gif"></h1> is meaningless to them because they can't read the text in the image, however you can use an image replacement technique to use a background image that accomplishes the same thing as <h1><img src="myimage.gif"></h1> except you write it as <h1>My Header</h1> and let CSS put the image in.....
I'm not terribly good at explaining it, google around a bit and find someone who is more articulate than me