Your MB will determine if you can use ddr3 or ddr2. DDR3 is no doubt faster but again its not the speed of the ram that is important its how much of it we have as any ram is a ton faster then disk.

DDR2 will be slower then DDR3 but you will have better fps with say 8G of DDR2 vs. 4G of DDR3, for example.

DDR2 is fine, whatever performance difference there is, for wow, will be minor. Be carefull on priceing a lot of times the older slower stuff costs MORE then the newer stuff cause they are making tons of the new stuff and few or none of the old stuffs.