What is the most memory you have ever seen in a classic Amiga?

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I'm curious about the Z3 Fastlane. Firstly, how many of the upgraded ones are around, the ones that can take 256mb? What I mean is, are these rare boards or can you find them second-hand in similar quantities to DKB boards?

Secondly, assuming somebody found one of the 256mb versions, would these SIMMS work? They seem to be sold for old Mac computers. They are 5v SIMMS, what voltage SIMMS are used in the Z3?

[X-ray]

@ Lemmink

That means that if someone has a Z3 with upgraded chips he can sell that for quite a bit. Is the access speed on that the same as the DKB (assuming same speed SIMMS)?