The place I like to use is zipzoomfly.com - they are fantastic to work with and they have loads of choices that look to be right up your alley
I recently did the same thing and ended up with a nice system just for rendering for $1400 using parts from them
I would agree that the processors, memory and drives are the three to get the most punch in - also remember that the power supply for the box is going to have to be a lot higher than a standard box
