My friend's computer is a HP Pavilion a134On. It came with a stick of 1GB PC-4200 DDR2 (533) memory. He just bought another stick of ram to go with it. Only difference is that it's 667 mhz. Shouldn't these two be compatible and just run at the speed of the slower chip? They aren't working together. BIOS only says there's 1GB of memory despite both modules being in the two slots. If I take out the new one, the old one is still recognized. If I take out the old one, the new one is still recognized. I did this for both slots. Both slots work and both modules work, just both modules don't work together. Why is this? Is there anything I can do to get the computer to see them both??