There are probably a few things you should look for. Color range is probably the most important factor, and a monitor that is at least capable of representing the [Adobe RGB Wide Gamut][1] color space (or [color gamut][2]) is critical. Many photographers like to work in the [Pro Photo RGB][3] color space, which is quite a bit larger than Adobe RGB, and often beyond the range of most monitors. If you want the ultimate in color quality, the higher the color range the better. 

Second, you will want a monitor that offers a high range of contrast. The color gamut that a monitor supports (such as Adobe RGB) clearly represents color range, but does not always represent the full contrast range. A high-contrast screen will be useful in representing a broader range of tones. I would look for something that offers at least 700:1 to 1000:1.

Finally, the brightness of a screen can play an important role. If you intend to place your computer in a darker area, a moderate brightness of 300cd/m^2 will probably be sufficient. However, if you need to place your system where bright glare from lighting or particularly sunlight might overpower your screens brightness, a brighter screen will probably be necessary. I would go for something around 380-400cd/m^2 if you have a brighter work area.

Once you have purchased a monitor, to get the most out of it, it is important to color calibrate it. Most monitors provide a default color profile that does not accurately represent colors...often by oversaturating. Additionally, the setting in which you use your monitor changes the appearance of the light it emits, so even if you manage to find a properly calibrated monitor, it is only going to be calibrated for its previous environment, not the one you place it in. You can easily calibrate your own monitor with a device like the [DataColor Spyder][4]. There are a variety of versions of these types of devices, and they range from $30 up to many hundreds or thousands of dollars. Generally speaking, something along the lines of the [Spyder3Pro][5] @ $170 is sufficient to accurately calibrate your monitor. If you spend $1000 or more on a screen, $170 is a drop in the bucket to get the most out of your new beauty.


  [1]: http://en.wikipedia.org/wiki/Adobe_Wide_Gamut_RGB_color_space
  [2]: http://en.wikipedia.org/wiki/Gamut
  [3]: http://en.wikipedia.org/wiki/ProPhoto_RGB
  [4]: http://spyder.datacolor.com/index_us.php
  [5]: http://spyder.datacolor.com/product-mc-s3pro.php