[realdan]

it could be this...the two ram are different as one Module Size 1024 MB (1 rank, 8 banks) and the other has Module Size 1024 MB (2 ranks, 4 banks) ..though this could be more an issue in the case of running in dual channel mode..dont know whether the chipset has a limitation in supporting 2gb if the clock speed is underclocked to 533 from 667 or it is of a different make even if you are not trying to run in dual channel...but looks like your motherboard cannot recognize beyond 1gb goes to show that it wont support 2gb memory of 667 speed probably in dual channel becos both chip are different..

maybe it try to run in dual channel mode when connected and sense memory difference so do not recognize the other memory module.
this looks more likely

or it cannot underclocked to 533 from 667 in dual channel
or it cannot underclocked to 533 from 667 when two slots are occupied