Inspiron 1525 Memory Upgrade

​I tried placing a 2GB DDR2 SODIMM PC5300/4200 (667/533MHz) stick into my laptop and it did not work for long. The laptop origianlly came with two 1gb sticks. I took one out and had replaced it with the 2gb stick above. It worked for a little while but then the system would just keep rebooting and telling me there was a fatal internal memory error. I replaced the 2gb stick with the original one and all is fine again.​

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​Any suggestions? Did I buy the wrong stick? Do I just need two of the same?​

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​Thanks in advance for any help!!!​