[jQuery] Position div accounting for screen size/resize

[jQuery] Position div accounting for screen size/resize

Can you change a position of a div, based on screen size/resize with
jquery? Something like
if
browser>= sitecontainerwidth
{
left:50%;
margin-left: 20px;
}
else
{
left: 0%;
margin-left: 600px;
}
I have a centered layout
div{margin: top auto;}
with a BG image.
There's another div
with a background design
that needs to overlay
on the BG, but off center.
Figure of what the problem: http://www.kacevisual.com/files/Div-difficulties.gif