its a memory thing. DDR stands for double data rate, and it means basically that. It can perform a read and a write at the same time, giving it basically double the speed. Basically, DDR 1 deals with speeds 100 Mhz, 133, 166 and 200 Mhz (or DDR-200, 266, 333, 400) and DDR2 just deals with higher speeds. 200 Mhz, 266, 333, 400 (DDR2-400, 533, 667, 800).
You need to get a DDR speed that corresponds to your chips FSB for best quality