According to the Apple tech notes (avilable on the developer site). The 12" PB has one soldered 128MB DDR266 (PC2100) SO-DIMM on the logic board and uses DDR266 (PC2100) on the expansion slot.
On the iMac 1Ghz side, one internal slot should use the standard DDR266 (PC2100)DDR RAM and the user-accessable slot uses the DDR266 (PC2100) SO-DIMM.
Meanwhile, a DDR333 (PC2700) should work these models.