A week ago i somehow managed to buy fully working HP Proliant DL360 G7 with 2x Xeon X5660 and 48GB RAM for 5 euro, and now i dont know what OS to install, because Windows Server 2012 R2 (which i already installed) dont have enough important drivers to work correctly (like ATI ES1000 driver), about Windows Server 2008 R2 i am unsure, and i havent any experience with Linux server operating systems. What is the best system now for that old server?
Are you intending on running this as a 24/7 machine?
Keep in mind, these are power hungry processors that don't idle down particularly well that have overall performance of a potato.
You had mentioned euro in your post which tells me that you likely pay quite a lot for your electric. These machines are dumpster fodder except for occasional use as a learning machine, definitely not worth running 24/7. Especially when a Intel box from the last 5 years will out perform it on significantly less power.