Hi LucaMauri,
I had a Z600 with a Quadro 4000 and had the exact same problem you are describing. The computer would sit quietly but as soon as any video or 3D graphics appeared on the screen, (flash, Youtube, vlc media player, etc.), the fan on the 4000 would rapidly ramp to full speed and eventually freeze the Z600 and cause the video card to shut down (and display to go black).
Before you give up on the Quadro 4000, try the following;
1. Remove the card from the Z800
2. Remove the plate that surrounds the fan and covers the heatsink, (you don't need to remove the heatsink).
By this time you will hopefully have found a small cat or dogs worth of hair, (PC fluff to techies), blocking the vanes on the heatsink and stopping the fan from doing its job keeping the card cool.
3. Remove the PC fluff. (An old paint brush with the bristles cut short are great for cleaning the vanes on heatsinks and fan blades).
4. Re-fit the heatsink plate and re-fit the card in the Z800.
You may just have a similar problem as I had and its worth a try. You never know it might be a simple fix. (When you think about it, the Quadro 4000 was launched in Nov 2010 so the card has been around for a while).
I know this issue isn't just related to the 4000 as my friends Quadro K4000 had the same issue about a month ago. The same fix worked for him and he was also as equally as embarrased as I was on how much hair/fluff these things attract.
After re-fitting my Quadro 4000 in my Z600 the difference was incredible. Super quiet at idle and playing video. It was like a brand new card.