In VirtualBox v3.2.4, only Mac OS X Server (and its 64-bit version) is listed as the available version for Mac OS X guest OS.

This is becuase of the fact that Apple's software license agreement only allows virtualization of Mac OS X Server on Apple's hardware (but does not matter if you are running Windows or Linux etc as the host OS). As a result, the GUI has to be modified appropriately so that it follows the software license agreement.

Technically, it should make no differences for you to select "Mac OS X Server" when you want to install Mac OS X.