DIMM stands for Dual In-line Memory Module, and refers to the packaging type of the memory - You may remember SIMM memory a while back, obviously Single etc etc
DDR, DDR2 and recently DDR3 have different contacts, and therefore cannot be mixed. If you have DDR now, then DDR you will need.
The 400 refers to the clock speed the memory is rated at. In this case 400MHz. This is sometimes referred to as PC-3200.
~S~ Jabo