dit: If I add any more SIMMs (tested with 32MB and 128MB, all I have at hand) it fails to boot and gives the sad chimes.
IIRC, bbraun also had a ROM modification to increase the upper memory limit so that it could use the full 520 MB possible. It may have had an issue where 264 was usuable by the system and the rest was available as a RAM disk. I can't remember that far back. There was a nice thread on his work regarding RAM in the Quadras about 10 years ago or so.