The MMU (memory management unit) page table is limited to 256K. When you install 256MB of RAM in a Quadra 900 or 950, the MMU page table grows a little too big and overwrites the RAM disk portion of memory. Replacing one bank of the 16MB SIMMs with 4MB SIMMS (reducing the total RAM below 256MB) will allow the system to work correctly with the RAM disk enabled.
Note that Apple does not officially support 16MB SIMMs in the Quadra, although in most situations they will work fine.
Seems about right.