I'll give you a pretty short cliff notes type on buying lcds.
Find out what panel type you want.
-TN's are the lowest grade, the highest is s-ips.
Find out what res you want.
-17", 20", 24" 30", any other size lcd has the same res as I mentioned before, just bigger pixels. For example a 19" has the same res as a 17" just that the 19" has bigger pixels.
Find out if you want either a 6bit or an 8bit panel.
-If you cant tell the diff then save your money and get a 6bit.
-16.7million does not mean true 8-bit.
-Make sure if you are getting 8-bit, that its true 8-bit and not 8-bit dithered.
Price?
-TN's these days are fairly cheap. You can get a decent TN for $250-$300.
-S-IPS are around $350-$400+