Its all about tone and colour..... Apple displays (to date) are 8 bit, so only 256 tones per colour channel. Eizo are higher (I think 12 bit) as are Dell, NEC and just about every other 'real' display. Apple have not made a pro display since crt days when they used Sony Trinitron screens. Budget wise - top end Eizo, bottom end Dell Ultrasharp. Check out dpreview forums or luminous landscape forums for this.... or google search - it's a popular topic.
Also - in my view AdobeRGB is important. You want a range of colours that will be more in line with what modern cameras can capture. sRGB (shitRGB) is the colour space of old crt TVs, and not up to spec with what our printers are even capable of outputting. Unless you are only making work for screens (inc tablets and phones) then you will want something more than sRGB.
Something else the NEC and Eizo offer is the ability to calibrate the screen, rather than simply profile it. This means you can access the display 'signal' (calibrate), rather than the video card 'signal'.
A great site for details on this stuff is
http://www.josephholmes.com (I'm a big fan of his Dcam profiles, number 3 in particular).
Anyway - lots of info for you. If you go for the Dell you should be pretty happy.