I am getting a new motherboard that takes DDR memory. Its been awhile since I have upgraded hardware and I am out of touch with DDR.

First- is it backwards compatible? Can I put my old DIMM's in the DDR slots?

Second- what do all the numbers mean? I see 1600, 2100, 2400, 2700, 3000, 3200, 3500, 3700, 4000 and 4200. How do you tell what kind works with which motherboard or how do you decide which one to buy?