While the spec of launching 20 tons to ISS seems impressive, how much of
its is dead weight (ATV) and how much of its is usable cargo ?

Cargo capacity 7667 kg
http://www.esa.int/esaHS/ESA4ZJ0VMOC_iss_0.html
(which also has a breakdown of the 20 tons into spacecraft,
consumables, cargo, etc)

Progress M 2350 kg
Progress M1 2230 kg
Shuttle Pressurized (MPLM) 8605 kg (includes rack structure)
Shuttle Unpressurized (ULCs) 9449 kg
HTV Mixed 6000 kg
ATV 7385 kg
http://www.spaceref.com/iss/logistics.capabilities.html