You need to keep CPU response delay to under 5ms per container and you will be in great shape.
OpenVZ works great and is easy to manage containers. If you prefer a full bare metal hypervisor I suggest KVM. Proxmox 2.3 packages both options nicely with a decent GUI that will allow newbies to get into deep poop.
I’ve used multiple Asterisk servers running well under VMware as well. And ESXi 5 is free to install and run. The vSphere client makes it pretty easy to manage as well although that only runs on Windows unfortunately!