CiviCRM Community Forums (archive)

A bit hard to tell without more details about server CPU and memory allocation, how many sites are sharing the server, etc. It sounds promising / notches above a cheap shared hosting account - which is good. You could ask for some references of other folks who are hosting Drupal / Joomla / CiviCRM with your host (or at least other PHP / MySQL based web applications).

Typically, a semi-dedicated server guarantees your virtual server certain resources, typically a portion of CPU time (eg minimum 1 out of 8 CPUs, burstable up to all 8 if none of the other VPS on the physical servers need CPU time). This helps because some ISPs oversell VPSes on their servers, and so even though the specs look good the performance can be poor for a relatively heavy application like CiviCRM. But there are many, many things that can affect server performance, including the speed of each CPU, whether it is 32 bit or 64 bit, the speed of the disks and their latency, etc. It's easier to summarize this by finding out how other CiviCRM users have found a hosting offering performs than to deal with all of the variables. Server performance is only one of several variables you want to consider, including reliablity (uptime), responsiveness and knowledge of support staff, etc.

A cluster means that there are several servers sharing a load. It doesn't really deal with whether your virtual server will get the resources it needs, since too many things can be added to a cluster for it to perform well. However, having a cluster usually makes it easier to add additional computing resources by adding more physical servers when limits are being approached.