DDR400 (PC2100) will be for an AMD Athlon XP 3200 (I'm not sure where it starts, you have to look at the clock rate on the package); DDR2 (NOT DDR200; DDR and DDR2 are two TOTALLY differant TYPES of ram) is for newer (and most 64 bit) motherboards.

DDR ram is however backwards compatable, so you can buy DDR400 and just let the system select what you want..
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