PPI = pixels per inch - a measure of screen resolution.
Generally speaking, the higher the PPI/resolution, the better the screen will look, particularly for things like watching movies or looking at images. On the other hand, higher resolutions are more expensive and all those pixels require power which means a bigger drain on the battery.
540x960 is not a particularly high resolution, but I've had phones with lower resolution and they've still looked pretty good to me.
Whether or not a particular screen is 'good' depends on a variety of factors, PPI being one, but a large part of it is also personal taste. Your best bet is to go to a store and take a look at the screen, see if you like what you see.
