Its a V-brake isnt it? The best cable you could fit would be a Gore Ride-On, which is sealed and should last as long as the bike without ever needing lubrication. I'f you also change the pads for Shimano or SRAM/Avid you should get the maximum performance from the brake.
IIRC they are Tektro, so essentially good brakes, however the rim may provide a poor braking surface, in which case you may never get acceptable power from it.
My dad has ordered a 906 City S, and no doubt he'll ask me to tweak his setup on that so i'll report back on the brakes after i've had a chance to fiddle with his.