Question : Problem: Problem adding RAM memory to PC

I am trying to add RAM memory to a PC with a Gigabyte motherboard GA-7N400 Pro2. The computer has 4 memory slots. It originally had two memory SIMMs 256MB each. I tried adding another two SIMMS of 256MB, but they are not recognized (no error received) and the computer still only recognizes 512MB of total RAM. When I took out the original 500MB and put in the 2 new SIMMS, it recognized them and the system memory was 512MB. How do I make the system recognize 1GB of RAM memory? Here is a link to the motherboard: http://www.gigabyte.com.tw/Support/Motherboard/Manual_Model.aspx?ProductID=1678

Answer : Problem: Problem adding RAM memory to PC

there is a compatibility tool on http://www.crucial.com

this is the models that is compatible:
http://www.crucial.com/store/listparts.aspx?model=GA-7N400%20Pro2

check if the one you bought is in the list