Installed 4 guess OSes on a ML 150 machine with dual-quad-core Xeon and 8GB of memory.
Running very smooth and even the fan has no noise! Perfect for demonstration use.
The 4 OSes: RHEL4, RHEL5, Win XP, Win 2003, all running on top of RHE5!
It is really amazing, hope I can have a machine capable of this kind of action later.