I suspect that, contrary to what you thought, you WERE operating in dual
channel mode. Your own results strongly suggest that.
I was only speculating that ram size was not a factor [above some
threshold]. Another poster says explicitly that it's not, but that
speed (which you cut in half ***IF*** you were previously operating in
dual channel mode) is the basis for the memory score measurement.
Keep in mind that the score is the score, but in real use of the
computer, it may be irrelevant: Doubling memory size from 1GB to 2GB
might not increase your score, but it may very well increase the actual
speed of the computer in SOME applications. On the other hand, once you
get to 1GB, for many applications, additional memory doesn't do much.
Similarly, for some applications, the difference between single and dual
channel operation is nearly double, while for other applications, it's
nearly zero.
Still, the "right" way to deal with memory on any system that supports
dual channel operation is to install memory in matched pairs so that
dual channel operation is enabled. You never gave us the exact
motherboard you are using, so I can't say with absolute certainty if
your system does or does not support dual channel memory. However,
empirically, your results suggest strong that whatever you believed, you
were in fact running dual channel before and are now running single
channel.
The original RAM came in pair and I spent lot of time making it work
in dual channel mode but I could not. So the sticks have been used in
*single* channel mode since then. I thought RAM size must be a factor
among others to determine the score but you indicate otherwise if the
size gets above 1GB. If that is the case then instead of getting
another 2GB stick for $300(The mobo requires REG/ECC) I just forget
about upgrading RAM and will regrettably get rid of the 2GB stick at
eBay.