With today's big disks and extremely powerful CPUs out there, memory is probably the more expensive resource.
When it comes to Virtual Hosts, also CPU power could be a problem, I have been running this site at Linode for a year now with no big problems, the only ones were because of some lack of memory, which caused the CPU usage to scale up, until I needed to restart the server.
Because of that experience I was investigating about vmstat. you can read vmstat man page at any time.
First let's check how memory works in Linux, memory is made of two components, the physical memory and swap memory (Disk), as you may know the disk is slower than the physical memory of your server so we all want to avoid memory paging or memory swapping.