The isssue is I don't mine a single coin. I mine a vast variaty and the big profit coins pretty much require windows and you can only chase them for a small amount of time. It is not convenient for tuning new cards with conf file. If you are mining ETH they wont be good unless they are 1060/1070. I mean they will be higher than what you have now but they are not worth ti for the cost other than 1060/1070. Is better things to mine on Nvidia.

BTW you don't need to pay for a key. You can happily mine away on an unactivated windows no issues, plus any old windows7 key still works and you can pretty much get them for free.