Basically DDR2 offers more bandwith, at a slower speed compared to DDR, which offers less bandwith but at a faster speed.
However with AMD's transition to Socket AM2 which only utilizes DDR2, and with Intel already being totally DDR2 based, DDR2 is the way of the future. Soon the DDR2 timings (or speed), will fall to the speeds of DDR (hopefully, or at least close!) and then DDR2 will have all advantages over DDR. So DDR2 is what you want.
You kno wut i say..... F*** ddr2, skip to DDR3......
DDR2 has higher latencies but faster bandwidths (DDR goes up to 600mhz?..... normal 400Mhz, DDR2 anywhere from 533Mhz-1000mhz). DDR2's high latencies effect AM2 alot but effects Conroe very little (which adds a plus to conroe). As boarder said, DDR2 is wut you want since its pretty much "the new tech" for RAM at the moment..... till DDR3