What is the difference between PIIX3 and ICH9 System chipset options in VirtualBox?
Before VirtualBox 4.0, PIIX3 was the only available option here. For modern guest operating systems such as Mac OS X, that old chipset is no longer well supported. As a result, VirtualBox 4.0 introduced an emulation of the more modern ICH9 chipset, which supports PCI express, three PCI buses, PCI-to-PCI bridges and Message Signaled Interrupts (MSI). This allows modern operating systems to address more PCI devices and no longer requires IRQ sharing.
Using the ICH9 chipset it is also possible to configure up to 36 network cards (up to 8 network adapters with PIIX3). Note that the ICH9 support is experimental and not recommended for guest operating systems which do not require it.