I just want to report back on this thread because I think I did indeed find a compatiblity issue once I tried to add in the older two 256 MB memory sticks. At first, it seemed fine and it appeared to work even faster. However, the first time I tried to use my Windows Media Player 11, the music would not play right. It was skipping/chirping and full of static sounding noise. While you could still hear the songs playing, it was just not going to work with all that background noise. This would occur with CDs and digital files on my PC so it was not the CD surface or the CD drive.
I ran the Windows disc error clean up and it seemed to correct the problem but that only lasted about an hour before the noise came back. I checked all the wiring and tried different speakers, but no change. I downloaded all the latest Windows Media codecs. Again, I thought THAT fixed the problem, but that only lasted about an hour until the rhythmic skipping noise returned.
So, I gave up and removed those two 256 MB memory sticks and the problem has not appeared since. I can only conclude at this point that adding those two in caused some kind of problem that affected WMP. I thought I should pass this along.