The Power Mac G4 computer uses PC133 compliant, 168-pin SDRAM DIMMs. The mechanical characteristics of the DIMM are given in the JEDEC specification for the 168-pin 8-byte DRAM DIMM. The specification number is JEDEC MO-161; the specification can be found by using the search string MO161 on the Electronics Industry Association's website at
http://www.jedec.org/DOWNLOAD/default.cfm
The electrical characteristics of the DIMM are given in section 4.5.6 of the JEDEC Standard 21-C, release 7. The specification can be found by using the search string JESD21-C on the Electronics Industry Association's website at
http://www.jedec.org/DOWNLOAD/default.cfm
The RAM DIMMs are required to be PC133 compliant. Information about the PC133 specification is available from Intel's website at
http://developer.intel.com/technology/memory/pcsdram/