Dual channel, crappy dimms?

It could be that the motherboard does not support both DIMMs together. It may have timings and speeds and voltages that are simply incompatible between the two of them and the Motherboard. The motherboard can work with either of them, but with both it does not have an option that each will work with.

And FascistNation is pointing out something I had not considered. You may want to try one of the methods pointed out in this page to verify that the system is not showing one DIMM.

http://www.computerhope.com/issues/ch000149.htm

I know memory is being used, but shouldn't system information show the total of the two dimms, physically installed in the 2 memory slots?

I've never had a computer there was less "Physical Memory Installed" than was actually installed,unless a dimm was not registering.

[quote user="philetus"]....With both dimms in, Sis info shows: Total Physical Memory of 512 MB and 469.43 Available .....[/quote]

System is using full memory.  You have about 500Mb of OS and startup apps loaded into RAM,and another 500Mb available for use.

Undoubtedly,  2Gb would show somewhat less than 1.5Gb available...and would be a marked improvement for XP SP3.

From my first post

                                                 " With 1 dimm in either slot"

Your first post

                                                 "Try the RAM one stick at a time"

My second  post

                                                 "Each stick,by itself, in each slot"

Your second post

have you tried them in different slots?

Try the RAM one stick at a time, you might have a bad one causing it to only show 512mb.

[/quote]

Did that. Each stick,by itself, in each slot and both sticks (alone) show 503.48 MB

Try the RAM one stick at a time, you might have a bad one causing it to only show 512mb.