If the manual states that it only supports 128Mb, 256Mb and 512Mb sticks, and in any combination between them, you can NOT use 1Gb sticks...
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There would be no difference between using two 512Mb sticks or using one 1Gb stick (provided it does support 1Gb sticks of course!).
Unless the motherboard also supports
Dual Channel Memory. If it supports that then you will see a difference in speed between using two 512Mb dualchannel sticks (faster) and one 1Gb stick (slower).
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