Running XP SP3, ASUS P5B Deluxe mobo, with two 1 GB Kingston HyperX memory modules. After confirming with Kingston and ASUS as compatible, I bought and
installed two 512 MB Kingston HyperX memory modules ( all the specs identical to
the 1 GB modules ) in the second channel slots on the motherboard, trying to get the maximum total 3 GB memory the mobo permits.  When I reboot, the screen stalls at the line where the system memory detected is displayed.  When I remove the  512 MB modules, it boots normally.

Am I missing something in the upgrade process? I thought the BIOS would detect the
added memory, without any tweaking in it's Setup.  Or any other 'manual' adjustments. I have not yet tried installing the new modules in the first channel on the mobo. If that works then it would confirm the memory is functional. Then maybe it's bad secondary memory channel slots.

Thanks for detailed technical suggestions to resolve.

   I would think that either the memory card is bad or your motherboard does not handle that much memory.