I'm having same problem as others. Manual says DDR2-533 is OK but 2x1GB are recognised as 2 x 512MB. Also turning off doesn't work; PC just re-boots. My MoBo only has slots for DDD2 - not 2 for DDR and 2 for DDR2, so is not quite like ECS Website Manual.
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Are your modules dual-sided (16 chips) low-density? If not, then they'll be recognized as 512MB modules.