What matters more than CPU and RAM is your HDD. Not how big it is, but how fast it is. Most online hosting providers use cheap, desktop class 7200RPM HDDs. These disks offer a ton of space but not a lot of I/O. In some cases, people will actually get better performance from properly designed cloud VPS systems (that run on SAN storage) than they would if they ran on a dedicated server. The best thing that I have been able to come up with in terms of general numbers is that a single 7200RPM desktop class HDD can handle up to 1000 simultaneous users at a time (approximately 100,000-500,000 users per month). More than that and you start to get high CPU utilization because it can't keep up with the disk I/O (resulting in really slow sites). Also, this does not take into account the OS and application overhead so your real numbers could be a lot less.