virtualbox.org

I know that VB emulates a single CPU in the guests. My host is a Socket 478, Intel P4, 3ghz CPU, which supports Hyperthreading.

I don't know if its just a glitch or what, but even if the guest sees one virtual CPU and I give the guest something to think about for a minute or two, I notice that both CPUs the host thinks it has seems to get used. Does the Virtual Box software itself use as many CPUs/Cores as the host has, even to run just one guest?

If I had a quad-core machine running Virtual Box, would I notice a significant increase in the guests actual processing capabilities? Is Virtual Box itself multi-threaded capable?

VB will use only one CPU at max for one VM. The Host can swap it's processing to either core, or distribute it over several cores. It will never go above the total use of one core.