I've got a new PC, and it's really really fast!! :-) .. so - of course - I'm running Vista Business, 64 bit and installed Virtual PC 2007 on it a couple of days ago and was blown away by how extremely fast my virtual machine was running - in fact I could'nt tell the different between 'the real one' (host machine) and the virtual one!
So .. I set out to measure the difference. Using SuperPI mod v1.5 - download: super_pi_mod-1.5.zip (60.44 kb) - Here are the results:

As stated above - this is the "real" machine, which in short is a Intel Core 2 Quad Q6600 with 8GB Memory running Vista Business, 64 bit edition.

The above result is my IBM ThinkPad T60 laptop at work with a Intel Core 2 Duo, 2GB memory running Windows XP SP2, 32 bit
edition.
And now .. the cool thing!!

The above is a Virtual PC 2007 (64 bit edition) machine running Windows XP SP2 with 1GB of memory - again running on the Intel
Core 2 Quad Q6600 with 8GB Memory running Vista Business, 64 bit
edition.
Note the performance!!! - It's much much faster than my high-end T60 laptop!! - Why is this?! - Well it's due to the fact that my new PC supports Hardware Virtualization, which is then enabled in the Virtual PC 2007:

So.. if you're planning to use Virtualization on your workstation / desktop pc, make sure you get a motherboard / CPU which supports hardware virtualization.
Update!!
In order to make the test complete, I also ran the 1M test on my VMware image, running on my T60 notebook:
The VMware machine is 6.42x times slower than Windows XP running in Virtual PC using Hardware Virtualization.