Processors and memory
Virtualisation in the way Xen & VMWare do it isn't magical. If you allocate 10 virtual machines (VMs) to 10 tasks, and each task requires 1 GB of memory, then your server needs > 10 GB of memory. Similarly you're best to allocate a CPU core to each VM to avoid contention. Same applies to disk space. So although you maybe only have a single case and power supply, you still need to buy all the really expensive stuff (CPUs, sticks of RAM, disk drives) in quantity.
"Containerised" approaches (OpenVZ, Solaris Zones) are a bit more efficient in that they only run one kernel, so they can share CPU, RAM and disk space. However they are never going to be as flexible as full virtualisation (Xen, VMWare) because you can't run multiple operating systems alongside each other, and the separation isn't so strong.