For anyone who has a 128MB 9250 with a 64-bit bus (4 chips), here's a ROM that correctly reports the whole 128MB. It should work with a 256MB 128-bit 9250 card (8 chips) and correctly report the size, however it can't use more than 128MB and will freeze past this boundary (tested in OS X, not 9). I spent several hours trying to work this out for use of a 256MB 9000 (this time with a 9000 136 ROM) and despite correct VRAM detection, the openGL implementation in OS X for R200 family of chips seems to be where the limitation comes from.
The rom is modified from Mac elite's wikidownload for 128kb devid hack for 9250 rev 127.
The modification in question should apply to any radeon 9000 / 9200 family card rom to allow adequate detection of high-density chips, but that's futile for card with over 128Mb since it freezes when attempting to use greater amounts of VRAM. Someone might want to test under MacOS9 for this limitation, since my testing was all with Leopard. Any idea welcome.
I don't want to make a long post, so if anybody has questions, feel free to ask.
Of course, use the ROM at your own risk.