The header looks centralized to me...
One thing you can do which works in "most" browsers (but not IE6 or earlier... obviously... very little really "works" in IE6) is use auto margins.
If you have a block element with a fixed width inside a larger block element then setting horizontal margins to auto will centre it... For example...
<div style="width: 1000px; background: #844;">
<div style="width: 500px; background: #484; margin: 0px auot; "><p>I'm a fish</p><p>I'm a fish</p></div>
</div>
PS: In CSS, margin and padding (and anything else AFAIK which accepts a top, right, bottom and left set of numbers will also take less than the 4). Basically if you provide fewer than the 4 then the CSS parser will copy from the other side... example:
margin: 0px 4px 8px; As there are only 3 values they will be: top=0, right=4, bottom=8, left(unset)=4 (as it inherits from right).
So in my example where I said '0px auto' that means top and bottom are 0px and left and right are auto. Its shorthand...
[center]