The clock speed in processors matters much less today than it used to. Back in the days of the Pentium 3s and 4s, clock speed WAS the easiest way to compare CPU speeds, but that is no longer the case in this new age of multi-threaded, multi-core systems. Some CPUs do more work per clock cycle than others and some CPUs communicate more seamlessly between their cores to produce overall higher performance. You have to compare CPUs by real-time benchmarks and tests in order to properly compare, especially when you are differentiating between different brands.
Intel Core 2 Duo CPUs tend to outperform AMD's rival chips at the lower clock frequencies, but once you enter the 3+ ghz realm, the differences are far less striking. For example, my system has an Athlon X2 6000+ @ 3ghz and it is, in many ways, faster than the comparable 3ghz C2D. Some of the benchmarks in high end games are very slightly lower; some are higher. Overall, in my opinion, it is the best CPU for the price, since you get just as much power for less.
In my opinion, regardless of clock speed, AMD CPUs give you more bang for the buck. You might get slightly (nearly unnoticably) lower performance but they tend to always cost less. And just so you don't think I'm biased, the notebook I just built has a Core 2 Duo, so I DO have experience with both.