You'll need to find out what speed your RAM runs at. You find most newer systems now use speed3200 ram or DDR400 in other words.
You can find out by installing this program:
When you find out the speed, just get the same as the one you have.
Also what motherboard have you got? If its reletivley now,and I guess older ones can support 1gb anyway. For new board, the maximum goes to about 4gb I think.
I hope this helps