One thing that might be wise to decide now is whether your going for an AMD or Intel CPU, as well as whether to get an NVIDIA or an ATI graphics card...
As you can see from past debates (often flamewars), opinions on what is best for what do differ, and you'll get different advice from different people.
But either way, a budget of 5000 is huge and should pretty much garantee you very high-end desktop hardware. I might also advise putting aside some of that for a huge LCD display, and good speakers, possibly surround, with a subwoofer. Especially if you're going to use this for gaming or video/graphics work.