If you mean to center hte page via float:center, that property doesn't exist. I suggest validating your page, but along with what Dave said, your page has 100% width and there's really nothing for it to center against.
I'm not a fan of how you coded thist page, as it has many errors and if you want a QUICK FIX, this is what you'd have to do. On "#page_content_wrapper" change the margins to this.
Auto horizontal margins are the universal way to center block content across the parent/screen. It requires a width (which you have set) and the auto margins. Very simple fix.
Though please validate your page and consider restructuring it. It's not a very pretty sight :). Though it's your choice.
Yes before any Sitepoint oldies say so, I know float:center has been "invented"