Android doesn't specify a bare-minimum requirement for memory, just keep in mind what apps you plan on running.
Specify how much RAM will be allocated to your virtual machine when you run it.
Note that you should choose the appropriate bit type for the version of Android-x86 that you downloaded. Set Type to Linux, and Version to Linux 2.6 / 3.x / 4.x.
Click the "New" button, and name your new virtual machine however you like.
If you have not already created a VirtualBox virtual machine for Android-x86 yet, do so as follows: If you are using Android-x86 for debugging purposes, some binaries (gdb for example) are built for 32-bit architectures and will not support debugging 64-bit binaries such as the Android app host. Download an ISO of Android-x86 from here.