Right, you mean Core 2 Duo and Core 2 Extreme...
Basically you have to name some specific models of these processors, for instance betweent he Core2Duo E6600 and the Core2Extreme X6800, yes, it is just a matter of Ghz, the cache is the same and its on the same architecture/core (of course). BUT if you say, went down a few models in the Core2Duo range, say the E6300 (base line model) this only has 2Mb of cache where as the Extreme has 4Mb so there is a fair bit of difference...
Most higher end Core2Duos can be overclocked to achieve the exact same performance as a stock Core2Extreme X6800 with ease.
Was there anything specific you wanted to know?
dragon