I'm building a computer. What should I put in it to make it amazing?

I’m planning on building a very nice PC, something that will last me a long time, and something that I can upgrade easily if I ever want/need to in the future. I’m a gamer, I also develop large pieces of graphic design projects (ie. posters), and I have a very large music and movie collection. What components do you think would be the best of the best?

I’m willing to put down no more than $4000, and I’ve already picked out the monitor I want. Hope you guys have some cool suggestions.