Ddr2 And Ddr3 Slot Difference

Hi Guys,
Just a quick question... the differerence between DDR 1,2 and 3.... in terms of pin placement on the motherboard? and am i correct in saying there is no motherboard that supports 2 different types of RAM? also... i wanted to find out the different speeds of RAM eg: 400MHZ,660MHZ 1333MHZ etc.. does that change the compatibility with the motherboard? For example, DDR2 400MHZ is installed can you insert DDR2 800MHZ and it will work? and if so will it work together with the 400 or must you have the same speed with all your RAM?
Ddr3 vs ddr2

Ddr2 Ram Vs Ddr3

Newer generations of DDR memory are typically faster than older generations. If you can find two modules with the same speed rating, though, they will usually be able to transfer data at the same rate. For instance, DDR2-800 and DDR3-800 memories both run at 400 MHz and can theoretically transfer 6,400 MB/sec of data. The most obvious factor on how to differ DDR3 from DDR2 memory is appearance. But, that is not as simple as it might seem. Unless you can find the specific reference on the memory stick, you will need to look at the key notch.