Linux is not supported for Carrara.  So XP for 64bit is your only choice. Of course, Carrara isn't 64bit, so that won't help you exept being able to supply your machine with more then 4G of RAM.
  I have no idea what Qube is, so I can't comment on that.

Otherwise I agree with the others that a basic fast machine with a render farm would be your best bet. (And I'm envious of your budjet!!)

Good luck and happy rendering! :D