difference in performance: virtualbox os and local os

"Given the number of variables involved (everything from caching efficiency to what programs are running your host OS) it is practically impossible to know the exact difference theoretically. However, I would do the following to determine this experimentally without affecting my current install;

Overview:

Purchase an identical harddrive
to what you currently have installed
in your computer.
Remove the current harddrive you have so you don't mess up any of
your currently installed stuff.
Install the OS's that you are analyzing to that harddrive -- note,
they MUST not be installed in
different partitions and the drive
should be zero'd between changing
the operating systems (this more
closely simulates how it might look
under virtual box -- i.e. a fresh
drive).
Perform your benchmarking for each OS
Put your old harddrive back in
Fire up VirtualBox and install each OS, perform your benchmarking
for each OS.

Caveats:

If you are trying to do a high-quality comparative analysis,
you HAVE to do the comparison from
each individual OS ON the hardware.
Make sure you pick a good set of benchmarking software that doesn't
bias toward one thing or another to
heavily. And if you do bias toward
something heavily (RAM usage
perhaps?) make sure you get enough
samples that the aggregate data is
meaningful.
Ensure the software running on your host os is the same AND minimal
during your benchmarking process for
each OS you sample
I would recommend defragging your harddrive between installs of the
OS's under virtualbox, this will help
ensure the virtual disk will be more
contiguous on disk"