You can shuffle around your running applications in Windows 7's taskbar, but not the order of the window thumbnails it shows for each app. If you're using Google Chrome and want to set a certain window last, hit the Full Screen switch.
Commenter Freakshowtime tips us off to this little inadvertent hack. If you hit the Full Screen toggle twice on a window - that's the F11 key in most cases - you'll set that browser window as last in Window's thumbnail previews. That can make it easier to get to for those who tend toward the mouse, and it works no matter how you've got your taskbar window ordering set up — always combine, never combine or combine when full.
If you've got a more elegant system, we'll gladly take your tip in the comments.