It depends on your workflow/needs. I have 2 24" ACD LED LCDs on my Mac Pro.
Final Cut Pro:
Display 1 Timeline, events, projects, editing
Display 2 Movie in full screen
Photoshop CS5/6:
Display 1 Windows (Palettes, history, colors, actions, layers, brushes, etc.)
Display 2 Working Image(s)
When one display for work is sometimes needed, I'll run EyeTV on my second display and have news or such on in the background while I work. Elgato's EyeTV Hybrid is a great addition. A small USB device with a cable input for cable TV without a box. 1-99 and local OTA HD channels, DVR/set schedules for programs, export shows to any format such as iPhone and Apple TV, live streaming via WiFi or 3G away from home. Also has input for VCR's and such to digitize old analog VHS/BetaMax videos.
All depends on your needs. Unfortunately Lion isn't as multi-display friendly as was Snow Leopard (some debate it). Snow Leopard, in my opinion, handled 2+ displays in OS X much better.
Good luck!
