I'm no expert, but from what I've learned, there are generally three grade levels (and corresponding price ranges) for today's LCD monitors.
TN-based panels- lowest cost, not as good for graphics/photo applications, but good for gaming, motion, web surfing
PVA-based panels - mid price, incorporating many of the good qualities of the higher priced IPS and lower priced TN technologies - probably the best value between price and quality for many people.
IPS-based panels - highest cost... best suited to graphics/photo editing. Apple Cinema Displays are IPS. But the 20" display in aluminum iMacs are TN, dang it!