I fried my old socket A mobo last week and purchased a new one, with new cpu and Vid Card, made the jump to PCI ex. Amazingly it booted with my old HD without reinstalling Windows, which i will do anyway after doing some nack-ups.
Everything seems to be running fine except for one thing, the system detects only 512 mb of ram instead of 1024. In fact, at mem count on startup, it stops at 256, but windows says i have 512.
Are both modules from the same maker with same speed? You might need to manually set the memory size from CMOS. First go into CMOS and set to Default, then change your settings from there and see what happens.