[System] Optional: Processor(s) may be set above 1 if you have more than one virtual processor in your host system. It is uncertain whether or not this has any affect on Android-x86, be it positive or negative.
[Display] Optional: Enable 3D Acceleration may be checked. The Linux Guest Additions may need to be installed to get any benefit from this.
Note that you must install the Guest Additions on the guest VM to specify the VBoxSVGA or VMSVGA graphics controller.