I assume you're talking about native screen resolution of the output display; if the native resolution of the display doesn't match your chosen card output resolution (or at least isn't an integer multiple of that), things will look fuzzy when, e.g., 1 image pixel takes up 1.5 display pixels. You can get an idea of this by changing the output resolution on your current video adapter and display to, say, 1680x900.
If you zoom in to 100% it doesn't matter what the screen resolution is as far as the individual pixels are concerned; you'll be seeing the individual pixels on either screen resolution. Just on the 2560x1600 pixel screen you'll see more of them, so a larger part of your image. They'll probably also be smaller (assuming nearly the same physical sized monitor), so you may perceive things as sharper.
Don't assume you'll automatically get a 99% colour balance with a new monitor - if you want to get correct colours, you'll need to run a calibration tool (at regular intervals) against your display card and monitor.