I'd say it goes like this:
Fat Agnus - Commodore part# 8371 : can access 512k of ram
Fatter Agnus - Commodore part# 8372A: can access 1MB of ram
Super-Fat Agnus - Commodore part# 8375 : can access 2MB of ram ( this is the Agnus ECS machines like the A-500+ and A-600 have ! )
Normal Denise - 8362
Super Denise - 8373 ( this is the ECS Denise in A-500+ & A-600 )
Upgrading the Agnus will give you the ability to have more chipram, like e.g using a A501 ramexpansion as chipram instead of slowram, but will not let you do extra hires screens afaik.
For the hires mode you will need the Super-Denise, and I think also at least a 1MB Agnus chip to be able to display a super hires screen.