If you have a decent budget, I'd get an Intel Core i7 or Core i5. You'll want multicores for photo/video editing.
As for RAM, if you're on 64-bit, 4 GB should be sufficient. Otherwise 3 GB is the max for 32-bit OS.
Motherboard depends on your processor choice. Just make sure you match the correct socket (LGA775 - Core 2 Duo, LGA1156 - Core i5, LGA1366 - Core i7).
As for graphics card, get a nice one. It'll help especially in your media-related editing. I would recommend at least a Radeon 4850 or Geforce 9800GT. If you have the budget, I would get a Radeon 4870 or better.
Monitor...bigger is better for photo/video editing. So put any extra money into a nice 22-23" LCD.