cheaper/easier to replace the OS than the hardware.
C2D has almost twice the transistors as the original yonah core duo. Twice the L2 cache. Better battery life. 64-bit extensions get you: ability to address more that 4GB of RAM (but 640k should be more than anyone needs

), more registers, true 64-bit ints... I keep hearing that the 64-bit extensions will be required to play bluray or hd dvds. The best part: all this for the same price.
What might actually be worth waiting for is the Santa Rosa architecture. It will unleash the full 1066Mhz FSB of the meroms. I think it also introduces Robson - the flash memory disk caching that looks pretty hot...