Is there a way to run a VM hypervisor, such as VirtualBox directly, from say, CentOS? Let's suppose that I want a high-efficiency server, but it can host guests with GUI-enabled OS's. Additionally, this can be booted directly via a terminal in a non GUI-enabled Linux distro?
Look into using a VM Hypervisor like KVM or Xen. KVM is built into the Linux kernel (and Xen is scheduled to be at sometime in the future). You can manage KVM using a tool like virt-manager.