Memory Problem

I am attempting to upgrade a PII-233 computer. It has a Biostar M6LTA motherboard that the manual says supports 512meg of Ram.After install 2 (128meg) chips, I clock off only a total of 128meg. I have 4 banks for chips. The board manufacturer says this board does not support high density menory.I used PNY memory that is 100/66 clockable. What memory can I use to upgrade this to 512meg?
Tom