VBoxManage modifyvm VM1 -hwvirtex on VBoxManage modifyvm VM1 -nestedpaging on VBoxManage modifyvm VM1 -sata on -sataportcount 2 -sataport1 VM1.vdi VBoxManage modifyvm VM1 -ioapic on VBoxManage modifyvm VM1 -nictype1 82540EM
VBoxManage modifyvm -bioslogofadein off -bioslogofadeout on|off
A rule of thumb might be one core processor, 1GBB RAM, 1 Network Card, and 1 Hard drive for each virtual machine (to ensure good performance), unless your goal is consolidating a lot of machines (that aren't very resource hungry).
The difficult one is usually the Hard Drive - keep in mind that disk IO will exponentially become a bottleneck depending on how many virtual machines reside on a single physical controller/disk (and the VM disk read/write requirements).
Starting with version 2.2, VirtualBox will enable hardware virtualization by default for new virtual machines that you create (VT-x and AMD-V)
...which will improve performance by offloading some of the work that was done by Software Virtualization (virtualbox) directly onto the processor.
On AMD processors, nested paging has been available starting with the Barcelona (K10) architecture; Intel added support for nested paging, which they call "extended page tables" (EPT), with their Core i7 (Nehalem) processors.
"VirtualBox's virtual SATA controller operates faster and also consumes less CPU resources than the virtual IDE controller."
Enabling the I/O APIC is required for 64-bit guest operating systems, especially Windows Vista; it is also required if you want to use more than one virtual CPU in a virtual machine.
It makes a lot more sense to give your virtual machine a gigabit (intel) network adapter (than 10/100), especially if the host computer has a gigabit adapter.
-nictype<1-N> Am79C970A|Am79C973|82540EM |82543GC|82545EM