DDR2 IS INCHING close to the 600MHz clock /1200MHz DDR throughput/PC9600 grading - let's use the correct description after a complaint from a Micron guy - but are all these complex looking heat sink and cooling systems really helping you crank up the last MHz?
The argument is that, to overclock more on the bandwidth side, you have to up the voltage, and so increase the heat output of the DIMMs - and yes, huge, finned heat spreaders or liquid cooling or whatever are needed to take away that heat.
How much do they really help? Early this year, I had a 2x1GB set of Micron DIMMs, declared DDR2-667CL4 - standard fare without any heat spreader. There was also Crucial Ballistix DIMM set, same 2x1GB, with full heat spreader and, supposedly, same declared speed and chip type (both were out of the fabs in the same 4Q 2005). Guess what? both overclocked to DDR2-800 CL 4-3-3-10 at 1.9 volts easily, but higher voltages of 2.0 and 2.1 volts didn't help get better latency. Of course, the Intel 975X couldn't exactly do more than DDR2-800 on an Intel board anyway, so I couldn't push the bandwidth further.