@Effy
Don't get me wrong I am not criticizing you. I realise you could have gotten a higher price for the Mk3, butI still think you were a bit ripped off.
The CS Mk3 is better than the Mk2, but even the first CS060 was far better than the 3640.
Assuming the other guy wanted a better CPU card for his A4K or whatever, he would have had to buy one (probably for a good deal more than he paid you) then have to see about selling the 3640 which not many users would want.
As I see it, he got a very good deal out of your good self.
Anyhow, all that aside, if you want more RAM with a higher bandwidth, you need to find a replacement CPU card (you are now in that other guys position).
The A4K has a terrible memory bus that buts the brakes on any CPU using memory on the motherboard. This is why the virtuall all accelerator cards came with local memory slots - even the first warp engine 28MHz 040 cards (only 3MHz faster than the 3640) ran rings around the basic 3640 system.