It's currently difficult to find the 256K chips; other options would be the AT91SAM7S128 (128K / 32K) and the AT91SAM7S512 (512K / 64K). If you have a PM3 (whether you built it yourself or bought it from someone else), please take a look at it and tell me which chip is used on yours (this will be the 2nd-largest chip on the PCB).
For what it's worth, it looks like the firmware in SVN today would fit fairly easily in the 7S128's flash, but will require a little bit of optimization to fit in its 32K of memory:
$ arm-eabi-size obj/fullimage.elf
text data bss dec hex filename
43878 42176 32885 118939 1d09b obj/fullimage.elf
text and data both sit in flash (so, 86054 bytes); BSS sits in RAM, so we're using a bit over 32K of -- but it's only over by 117 bytes.
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