early iMacs are a bit finicky about 256MB SO-DIMMs, in that with some, they will only address the first 128MB of the SO-DIMM.

Click to expand...

The trick is to get ones with chips on both sides. These are "single banked". There's an upper limit on the size of the individual chips the iMac can address on the DIMM, so more chips = better.

Click to expand...

yea, low density chips, like you said double banked. the same limitation as the Beige G3 and B&W G3 and Yikes G4 (i think cause its really a B&W under the hood)

they would only see 128mb out of 256mb available of the chips if you used a stick with only 1 bank = 8 chips ( high density chips) , and would see the full 256mb if it used double banked = 16 chips ( low density chips). i am not sure how many chips are on the sodimm stick.