it has quite a similar function as my ages old ISA Trident or Cirrus Logic - to make a 2D display
This is completely wrong.
If you run the compositing Boing Ball demo from Hans De Ruiter, you will see the
clear performance difference between a RadeonHD 6870 and lower specced cards. The demo is much smoother and faster. If you run M.A.C.E full game with FPS in 1920x1080, you can also see the performance difference with RadeonHD 6870.
And A-EON has some software coming out in the next three months that will demonstrate some more of the power of the RadeonHD on AmigaONE X1000 over other AmigaOS 4 hardware.
Choosing lower specced second hand hardware may not be the best choice soon...