"ECS" referred to the Agnus chip.
OCS Agnus=512k chip
ECS Agnus=1mb chip (+ productivity modes, I think)
There is no such thing as an "ECS Denise". Denise only had 1 major revision (they added Extra Half-brite mode) but this revision was made before they shipped the first A500 & A2000's. Any revision Denise will work with any Agnus chip.