They're stacked at the top for me at 1680 x 1050, but I've intentionally left ABP off for this site only, and they're not really disrupting anything for me.
This is also how I generally feel for the stacked ads on a 1680x1050 resolution. It works and is not interfering too much.
The reason behind this display optimization is it does generate more revenue per ad display for the page. The header generated the most revenue. Placing it below the page title actually increases the value to advertisers of the header ad (according to adsense recommendations). So this is a 2 for 1. More ad revenue does equal more resources we can dedicate to the site expansion.
But I definitely want to make it work for the users rather than making intrusive ads that detract from the content. So I place very high stakes in getting this right. I never want to have BS ads that prevent you from seeing the content unless you watch a video. The only exception to that rule is if we integrate video or playing games more into the site since they are large sources of bandwidth consumption.