4
well for 1,
google_ad_width = 728;
google_ad_height = 90;
google_ad_format = "728x90_as";
you have an ad with a fixed width of 728px
your left column = 150px
your right column = 180px
728+150+180 = 1058px
which for 1 is wider than a 1024x768 display.
this difference with the ad will force the right of the page wider.
because the centre columns are set at 100% then anything in the centre column that's wider than 630px will force the screen wider on 1024x768 displays
i say 630px because even though your resolution is set at 1024px you have to allow for the scrollbars ;)
anyone using less than 1024x768 displays these days really needs to get a new monitor. i'm not saying that selfishly.. but the world is moving on and webmasters need to move on too.
if you want to cater for those users, you are really restricted if you don't want them scrolling right.. i suggest 2 column only for displays less than 1024x768, 3 columns for 1024x768 or more.
it's easier to optimize on a fixed width theme rather than fluid. but both have good sides and their downsides.
