Thanks very much for your suggestions, I just bought 2x2GB amicroe ram, so far so good, the only problem I found is that in system profile, in the memory section, it shows ECC is disabled, I don't know what it is, I'm not sure if it should be disabled or the ram has problem:
Memory Slots:
ECC: Disabled
BANK 0/DIMM0:
Size: 2 GB
Type: DDR3
Speed: 1067 MHz
Status: OK
Manufacturer: 0x8551
Part Number: 0x494D5348324753313341314631432D313046
Serial Number: 0x2A05CF15
BANK 1/DIMM0:
Size: 2 GB
Type: DDR3
Speed: 1067 MHz
Status: OK
Manufacturer: 0x8551
Part Number: 0x494D5348324753313341314631432D313046
Serial Number: 0x2A05D126