Difference between Core 2 Extreme and Core 2 Duo

The clock speed (GHz) and an unlocked multiplier are the only difference between both the processors. The Core 2 Duo is downward unlocked.

Core 2 Extreme:
4 MB of shared L2 Cache
1066 MHz front-side bus

Core 2 Extreme Quad:
8 MB of L2 Cache
1333 MHz front-side bus

Core 2 Extreme is made for enthusiasts and people who love to overclock.