Ok, the old sith would definatelly beat the borg, with ease..
I am curious about the rule of two though..
In this case it becomes very important wich 2 sith will be in charge at the moment..
For example, Darth sidious is strong enough. He can even walk among the jedi unnoticed. He is so strong in deception and cloaking that the borg wouldn't even notice him. Put this in combination with vader for example and you got yourself a win. Put darth maul in his team though, and you added a "worthless" fighter. Don't get me wrong, maul was one of the best in fighting. But his dark side was based upon aggression and not enough on power.
Take 2 from the "past" though and we wouldn't even be speaking of borg.. Darth Revan in combination with Darth Bane? Rrrr.. Although that 'rule of two' wouldn't last long..
EDIT: A non-possible situation ofcourse as there are, what, 3000 years between them?
Nah you know what, I still cannot see the borg overcome it all..