Thats because CPUZ reads the SPD info from the memory modules, so it indicates 2x256 MB. I have the same issue, its caused by the chipset-you probably use Intel 440 BX chipset, which just cannot use single sided 256MB modules. So, if your modules are 256 MB, both single sided, the chipset simply uses only one half of each of them, which means 2x128 MB. Thats it
I know what you mean, but this mobo of mine used to run double and single sided 256MB modules before without issue. Strange thing was I recently reformatted my computer and reinstalled Win XP and now it only detects one of them. Didn't use to be the case back then.
And I using VIA Apollo Pro 133A Chipset. Says here in the manual that "Supports single & double sided DIMMs"
Had to dig out my ancient mobo manual for that =P