I also just got a refurbished Z600 with 2x Xenon x5550, 24gb of ram.
I added a Quadro 4000, 240gb Intel SSD, 2TB WD Red Pro Hard drive.
A beast IMHO.
But I have an overheating problem. And now im desperately looking for a solution to cool down my CPUs. At full load it heats up to 100°C 
I noticed that the company that refurbished it, used non original fans on the heatsink
( https://www.dropbox.com/s/i39tcepxzxy9hou/CameraZOOM-270920161415.jpg?dl=0 )
Do you think are those the problem? They get really loud.
Im also looking for a water cooling solution, so any help would be apriciated!
Thank you.
M