the main beneficiary of ddr3 is the server platform , they have 12/18/24/32 etc memory modules running and ddr3 consumes lower power of a few watts but which adds up when u are using so many modules . and server power consumption is highly important in a 24x7 environment because of power cost/cooling issues as cooling itself takes up both space and power.
as an example of this performance -heat-space-cooling chain in a server environment is for example fermi , which nvidia will market as lower heat and power consumption and space requirement and much higher performance than say , a greater number of x86 processors .
for desktops , the gains of going from ddr2 to ddr3 except for compatibility are far less important than , for example , the gpu or cpu one is using . using only 2/3 modules the power saving isn't significant .
but again , in notebooks , lower voltage ddr3's will consume less , which will matter very much . the first ddr3's were 1.65 or even higher , 1.5v have come up recently , and 1.35 v are cropping up slowly and these will make a big difference in notebook where every watt , so to say , is important to conserve .
http://img.tomshardware.com/forum/uk/icones/wink.gif
if ur buying new , just get ddr3 , for compatibility and lower prices in the future , as ddr2 will only go up in prices from now and wont be supported on most of the new mobo's .
edit : even though u've selected the best answer AS i was writing the above , i guess now whether u'll even read this post .