I just installed and tested an memory module DDR2 SODIMM unbuffered DDR2-400 144pins. It's officially branded as Kyocera memory but works just fine in the Xerox Workcentre 6605DN model. The exact specification can be found here : 1GB Kyocera DDR2-400 144-pin SDRAM SODIMM Printer Memory (p/n MDDR2-1024)
I image I would have 1GB + 256MB available but only 1024MB is recognized. No harm done I guess.