>> 256MB cards seen as 128MB are still seen as 128MB card
>> (just tested on Peg2).

> Nope

Well, Kronos wrote that he actually tested it on his setup.

> if the Radeon has two seperate physical ram banks (128MB each),
> chances are that it will work. Mine it's this way and is seen as 256MB

Then yours is obviously one of those 256 MiB cards that are fully recognized by MorphOS (single 256 MiB aperture despite two banks), and not one of those 256 MiB cards that are seen as 128 MiB card (two 128 MiB apertures), which is what Kronos referred to.