You are misusing the hScale and vScale values... they are not supposed to be set to the size of your display, they are supposed to be set to a scaling value. They are typically a number less than 1.
If your horizontal display size is 1680 then your default horizontal scale would be 1680 / 2560 = 0.65625
The vertical scale is a little more complicated to calculate. It starts out the same way... 1050 / 1600 = 0.65625 but then has to be modified to make your screen elements square. A 1680x1050 display has an aspect ratio of 1.6 -- so your vScale is 0.65625 * 0.95 = 0.6234375
However, as was already pointed out, unless you're trying to change nUI's default layout, you don't have to mess with scaling at all as nUI does all of these calculations by default. It's only when you want to make the UI bigger or smaller than it is by default that you have to mess with this stuff. The complete details on how to do that are here:
http://www.wowinterface.com/forums/s...ad.php?t=20917