"There are two ways of installing VirtualBox: from
precompiled binaries that are available for some distributions and
come under the PUEL license, and from the sources that are released
under the GPL. This article will show how to set up VirtualBox 3.1
from the precompiled binaries.
"As of version 2.0 VirtualBox supports 32 and 64bit host and
guest operating systems (if you want to install 64bit guests your
processor must support hardware virtualization and, of course, the
host operating system must be 64bit as well)."