All 64bit read 16Gb and above.
Many voices are for windows 8.1 for better CPU utilization.
About Win 10 is still to early to tell but DX12 support will be for future games.
I use win 7 ulti 64 should I change it to other version like home premium 64 since ulti have some internet protocols that ill never use so my pc gets fastet?
What is your system, and what is it used for? You almost certain you need no more than windows Home 64, but your usage and the type of PC you have may change that.