The reason DDR3 is not worth yet is because CPUs runs at FSB speeds lower than that of RAM. Take for example a 2.4GHz CPU (quad-core) that runs at 1333MHz. It is running at a FSB of 333MHz quad-pumped:
4 x 333MHz = 1333MHz
In order for a system to run as stable as possible the CPU FSB to RAM Speed ratio must be 1:1. So in order for you to run 1:1 with the CPU at hand you must have RAM that runs at 333MHz. Because DDR is dual data rate (not sure if I have the correct meaning of the acronym) you will divide the listed RAM speed by 2. To match the CPU FSB:
2 x 333 = 666 (667MHz DDR2 RAM)
Although you can get RAM running at speeds much faster than that (667MHz), you won't be fully utilizing it, therefore your CPU will be bottlenecking your RAM.
HOWEVER
If you can manage to alter the clock/multiplier combination of you CPU and keep it stable you, can get better performance out of your RAM. Since I'm going a little out of my range here, for the sake of the concept, we'll say that (using the previous CPU) 7 x 333 = 2400 (2331 in reality).
To run stable at stock settings this is what you will have:
CPU Multiplier: 7
CPU FSB: 333MHz
CPU Clock: 2.4GHz
RAM: 667
Now if you mess with the multiplier and FSB and you get it stable, you could have something like this (assuming you will keep the same clock speed):
CPU Multiplier: 6
FSB: 400MHz
CPU Clock: 2.4GHz
Now what will this allow? Because you're now using 400MHz as the FSB you can get faster RAM:
2 x 400MHz = 800MHz
Lastly, by changing the FSB to a higher speed you can increase the performace of your RAM (assuming you bought something that is being bottlenecked by the CPU). But beware, you can't just pop in numbers and hope it'll work well together. To change the settings you will have to do it little by little to keep it stable, you might even have to raise the VCore (voltage? Again out of my area) to power everything, thus emitting more heat and thus endangering your CPU even more.