Most modern games include pre-scaled sets of graphics for the most common resolutions, e.g. 2560x1440 and 1920x1080.
However, if you're playing a 3D game at 1920x1080 on a 2560x1440 monitor, icons, user interface elements etc will look bigger. Also, the monitor has to interpolate to the nearest pixel which won't look quite as sharp as in the native resolution. The net effect is slightly less sharp/crisp elements and fuzzier edges.
Whether you can learn to live with that is a good question. If I were you, I'd pop by Currys/PC World and ask them to show you the difference, so you can see for yourself if it's worth it or not.