I currently have an E3-1270 v6 @ 3.80GHz - 32 GB Ram, approximately 2.6k online users with 70% peak CPU usage, this is a very good processor.
for over 150 users I would recommend: 2gb ram + 2 vcpu 2.0 ghz
or ram 2 gb + 1 vcpu 3.6 ghz or above.
With 300 users on this processor (1270v6) it doesn't even use 30% of 1 core, so the amount of Ghz and CPU processor architecture makes a lot of difference, so keep this in mind before choosing a processor.
High frequency processors have done better with me, so high frequency gamers and xeon processors are more interesting even if they have a smaller but higher frequency core, which makes the processor response "faster".