DDR PC2100 RAM vs DDR PC1600 RAM ???

[BroncoBoy]

Can DDR PC2100 RAM be used in place of DDR PC1600 RAM. I have one 128mb stick of DDR PC1600 RAM in my GA7DX mobo with the 200 MHZ FSB 1 GHZ Athlon Chip? I want to increase it to 256mb but don?t want to buy more PC1600 RAM because I may upgrade to a faster Athlon with the 266 MHZ FSB. Is there a problem mixing while I am still running the 200 MHZ 1 GHZ Athlon?

Is there a physical difference in the chips, or it just that PC2100 has been tested to and certified capable of running at the higher frequency?

Thanks in advance for the info.