097S03635 512MB Phaser Memory (1 X 512MB)
097S03743 256MB DDR2 Memory (1 X 256MB)
However, I have succesfully upgraded my Xerox Phaser 6130 with a 1 GB module. It was a module from my Toshiba Satellite P100 laptop. So most likely you can use 256 MB/512 MB/1024 MB modules with the following specs:
PC2-4200 DDR2-533 1.8V SODIMM 200 pin
PC2-5300 DDR2-667 1.8V SODIMM 200 pin
1 GB modules cost less than USD 50 these days.