64-bit VirtualBox host running a single 32-bit guest, good idea?
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This "emulation" word may stand in the way of your understanding what virtualization means. The CPU is not being emulated, nor is the operating system. In fact, I'm not sure if anything is actually emulated. Instead, what happens is that the virtualizer stands between the Guest OS and the hardware. This is because the Guest can't, under almost any circumstance, be allowed direct access to the hardware or filesystems for fear of leaving it in an invalid state for the Host. For instance, it should be obvious that you can't have two filesystem managers managing one filesystem at the same time. So, essentially the Host OS just runs as normal, and whenever it accesses hardware or filesystems or probably a number of other things, a request is made to the Host OS.
This is over simplified and doesn't address 32-bits vs 64-bits, but hopefully it gives you some insight into what is happening.
I run Windows Server 2003 32-bit as a Guest in VirtualBox on my 64-bit Windows Vista OS. I have a Core i7 CPU and 6GB of RAM.
Mostly what is emulated by VM software is the physical devices and memory management functions. With hardware virtual features such as in the Core i7 those functions are done in hardware through the use of nested page tables and hardware interception of I/O addresses.
For an X86 processor emulating an X86 processor the CPU usually executes the instructions and the virtual software intercepts some of the resulting accesses and operations. When a CPU is emulating a completely different CPU then it (usually) has to interpret the instructions and execute them. That is usually much slower.
On a multi-core CPU even a single-core emulated CPU may actually run faster than the real hardware. That's because the hard disk access and actual hardware I/O is done outside the virtual machine and may actually be handled by some of the other CPU cores in parallel.