This is DIY project, which adds 8 Mb of autoconfiguring fast RAM to your Amiga 600.
Here is complete archive including documentation, schematics, source files, etc.: a600_8mb.zip
Update! Contains 4Mb as well as 8Mb NON-autoconfiguring chip firmware. a600_8mb_addon.zip See READMEs included for more information.
Now I'm making new version of this board, however, it is still possible for me to order new unpopulated PCBs. I'll send you PCB for approx. 10$ plus postage, and programmed and tested CPLD for approx. 6$ plus postage. If you are interested, please contact me (email could be found in the abovementioned archive).