theoritically even 1 user with light traffic could see results with overclocking. lets say the router just gets 1 packet in, no other load or traffic. the non-overclocked proc can turn that packet around and send it out in 2 milliseconds. the overclocked one might do it in 1.2 milliseconds. so a lower ping and higher framerates for FPS's.
but, those are made up numbers.. is there a big enough difference to notice. nope.