if you want to fix the header width problem you can go to
#mainwrapper element and add
min-width: 1100px;
width: 100%;
min-width mean no matter what resolution enter your site will aleast with width 1100px if the resolution is higher than 1100px will auto set to 100% width
The header_bg.gif I dont know how you want it look
http://img353.imageshack.us/img353/9387/printot0.jpg
this is how your site look on my brower it is center and it look nice.
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