The suggestion by applefreak to move chips around is an excellent one. The power-on diagnostic routines are not comprehensive, so a "memory error" indication might actually be due to, say, a decoder failure. If the problem does not move with the chip, the memory chip is probably ok. If the problem does move around with the chip, you can feel confident that the chip is indeed bad.

Are all the memory chips socketed in your Mac? If not, then the advice above will be impractical to implement.