I run NGINX with PageSpeed and HHVM.
My system has 4GB RAM, 2 vCores (Intel Xeon E5-2690v2) and a 25GB RAID 10 SSD. I can handle with this setup easily 1.000.000 users a month, traffic was optimized (I now have a CDN) and so stay around 250GB bandwith for my website only.
I rather not add the firmware archive here...
It consumes more than 10TB traffic a month.
Before I forget it, usually 100-500 people are online. And I never experienced a resource shortage, actually it is often the case that the RAM is only used to 25% and the CPU to 20%.