Hi the motherboard is MICRO-STAR INTERNATIONAL CO., LTD MS-7502 (Socket 775), and the RAM in the pc is 4,00GB Dual-Channel DDR2 @ 332MHz (5-5-5-15) (it´s the original RAM).
The new RAM i tried to put in is 1GB PC2-5300U-555, MEU12864D4BC1EP-30R.
Hey does the board have more than 2 slots? If it does make sure it is in dual channel mode. Maybe the 1 stick or maybe port is faulty? I know of such cases.