Photoshop

That is a 6-bit monitor, which you don't want for serious image editing (if you can spot 3 shades difference on the RGB scale that is).

What you will probably want is as follows:
20" or higher (for the higher resolution)
perhaps widescreen so you can fit more stuff (unless you thing regular dimensions would fit your work process better)
IPS based panel (perhaps even AS-IPS) for viewing angle, which affects color uniformity across the entire screen surface.
Response time - believe it or not, this is also important for image editing; if it is slow, then while you are drawing with a brush, the brush is gonna have a different shade than what you are actually drawing as you move it around because the pixels are too slow the update as you move the brush.

What you will need to make a choice on:
glossy or matte finish - matte finish gives the screen a constant "dusty" look like there is dust on the screen making solid shades of color not look so solid; glossy finish alleviates this, but can have serious glare.

Things to consider for the real high end:
10bit monitors
ASV panel (very, very rare)

From a list of ones with known response times (as tested by xbitlabs.com)
AS-IPS NEC 20WGX2 20", widescreen, glossy finish, good response time
S-IPS LCD2190UXi 20", matte finish?, slower response time
S-IPS MFM-HT205 20", glossy finish?, good response time

This page also has recommendations you might want to consider:
http://forums.anandtech.com/messag [...] erthread=y