PE2950 and Linux virtualization

[Scott R. Ehrlich]

What are people's experiences with CentOS 5.0 64-bit installed on dual 3 
Ghz quad-core PE2950 systems with 32 GB RAM each, high-performance 
computing (applications that tax both the CPUs and RAM), not currently 
in a Beowolf cluster but could adapt to that, and doing so with VMWare or 
other vitualization software vs activity being done directly in the OS?

How much of a performance hit, or gain (I'd presume hit), does 
virtualization cause an application, resulting in what percentage poorer 
or better (I'd presume poorer) performance vs dealing directly with the 
OS?

It would be nice to have a VM perform some work, and if a person's code or 
application breaks, have it take down a VM while keeping a machine up, and 
not affecting other people's work.

It may also depend on if an application or code is written directly 
with/for the physical cpu/hardware vs more general use (VM).

Thanks for insights and experiences.

Scott