DDR is 184-pin. DDR2 is 240-pin. The "400" is the bus speed in MHz. You will also see "PC3200" and such in which the 3200=3200MB/sec bandwidth. So in the end you pick a CPU and compatible MB and get the ram spec'd highest according the the capabilies of the above. So if your CPU/MB runs a 266 bus you don't need DDR400 memory since it will simply run at the 266 speed anyway. If your combo supports a 400Mhz FSB then get DDR400.

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