is it possible to install VirtualBox on a non-intel Mac ?
My configuration : G5 with Mac OS X 10.5.8
No as VirtualBox does not emulate the CPU but virtualizies it as described by Oracle.
You would need to find Virtual PC or Bochs (I have used both in the past) or others in a list from www.macwindows.com
However now getting an extra second hand intel box might well be the easiest thing to do and would definitely provide the faster solution.
No you can't unfortunately. See the first line of the prerequisites on the virtualbox support pages here