As long as the PC2-6400 module is unbuffered (it probably is), AND both PC2-6400U and PC2-6400 modules have similar characteristics, then the two modules should be compatible. Only way to be certain is to try them together.
As long as the PC2-6400 module is unbuffered (it probably is), AND both PC2-6400U and PC2-6400 modules have similar characteristics, then the two modules should be compatible. Only way to be certain is to try them together.