Before changing BIOS settings we may want to see if hardware virtualization (VT-x for Intel, AMD-V for AMD processors) was already enabled.
From a terminal issue
grep --color vmx /proc/cpuinfo ## for an Intel processor grep --color svm /proc/cpuinfo ## for an AMD processor
If virtualization was enabled the flag vmx (for Intel CPUs) or svm (for AMD CPUs) will be colored.
If not, and the CPU supports hardware virtualization, we need to enable this in the computer BIOS.
See with the manual for the hardware and search for Virtualization, Virtualization Technology (VT-x), or similar, here shown for an Award BIOS:
Set this entry to Enabled in case it is not yet set.
We then can choose to use hardware virtualization in the virtual machine's System -> Acceleration settings:
By this we use hardware virtualization for optimal performance, and we would even be able to run a 64-bit guest OS on a 32-bit host for testing.