Can you run a machine emulator (Bochs) inside a virtual machine?
Long story short: yes.
Each virtual machine is technically "independent" of one another, and with VirtualBox, you could easily do this, since it is supported on both Windows and Linux host operating systems (emulated or not). You could simply use Windows as your "base-host" OS, run Linux in a VM, and then use that operating system as the new base-host for Bochs.
Do note that your only limitation here is your hardware. Depending on the requirements of your development, you may require more memory, or an upgrade to a 64-bit "base-host" operating system. That being said, if you choose your Linux distros wisely, any modern system should be capable of arbitrary nesting like this.
You will see improved performance if you enable x86 virtualization support on your computer (if your motherboard and CPU support it), and AFAIK, you can "pass through" this feature to multiple nested virtual machines. If we have n virtual machines nested in eachother, this is supported so long as the 1st to the n-1th nested guest OS has support for x86 virtualization (the base host must also support it). Do note that there are still some security issues to be aware of if you do this, so take the appropriate precautions.
I sometimes run a whole VMware vSphere virtual infrastructure within VMware Workstation, along with virtual Cisco routers and switches and EMC Celerra Virtualized Storage Appliance, for self-learning and testing purposes.
I call it 'Inception Computing', and it can get pretty confusing. It is however somewhat cheaper than using the real gear.
To anyone who reads this question, sees the accepted answer, and thinks this will apply to VirtualBox as well, please see comment from @superbatfish on the original question. He makes a good point about the difference between virtualization and emulation.
Specifically, "hardware assisted VM" will not be available in the "top-level" guest OS (that is, at the time of this writing, VirtualBox doesn't provide hardware VM in guest OSes). With VirtualBox, this means you can only create 32-bit "nested guest" OSes, even if the top-level guest is 64 bit and your "real" host has hardware VM.
May not be a showstopper for you, but it's worth noting. I needed this in order to run Vagrant on an Ubuntu guest inside Windows, for Chef cookbook testing.