I read all this topic a few days ago because I was looking for infos for a new computer for video editing.
I just bought an HP Z600 and a Quadro FX 4000.
Windows 7 64bit, 16Gb of RAM, 2 Hard disk of 1 Tb each one.
I wasn’t sure about the video card, I first ordered a GTX 580 (last model) because of the higher CUDA cores, but this card needs 2 cables of 6pin and 8pin from the PSU, that I don’t have on the Z600.
So I moved to the Quadro 4000.
I will let you know how it works.
Chris