It depends on what you're planning to do with your virtual machines.
If it will be for desktop use, testing and such, then VirtualBox is simply the answer... But if you will use it to act as a server that requires intense resources, you can use type1 (native, baremetal) hypervisor. Since you are using RHEL 64bit, it's either RHEV(KVM) or Xen...
More info on
virtual machines (type1, type2)