I'll write it's doubtful you mixed voltages.
And I'll note that it's OK to mix speeds if the newer memory is of a higher speed. You seem issues (many times) if you put lower speed than required memory in.
I have a Gateway P4, 1400mz.It came with 128 megs. I have added more-640 total. I asked sales people what I needed and installed. But they are different speeds and I think voltage. Is that a bad thing?