Clock speed is pretty irrelevant these days.
It's all down to the design of the CPU - the on board cache, bus and pipelines. There are also major re-works of the actual CPU architecture so it performs most functions way more efficiently at lower clock speeds (thus improving power efficnency and heat dissipation). The problem Intel had with the P3 and P4 was that they relied on clock speeds to sell their CPU's, which was BS because they were a crap architecture, and they performed like an american muscle car - all grunt and noise, but no real speed!
Dual core does double the clock speed. The problem isn't the sharing, as each core has its own L1 cache and bus on the chip dye. The problem and the reason that you don't benefit from the doubling of the clock speeds is that current operating systems are not designed to efficiently manage and utilise more than one core at a time.
You realise, AMD have always had lower clocks than Intel CPUs? For example, my Athlon XP 3200+ is only 2.4ghz. That out-performs an Intel P4 3.2ghz CPU in almost every benchmark. Well, it matches it at least! It won't win on raw data throughput for obvious reasons however.
The Athlon 64 X2's are also very low clock speeds. It's how AMD maintain excellent power/heat efficnency and their CPU architecture and bus size more than makes up for it.
As it stands, the current top and middle end Core 2 Duo's (Conroe model) outperform the fastest AMD chip - the FX 64 dual core - in every single benchmark there is available to modern computing. In short, they rock.
Power consumptionGame/3D benchmarksApplication benchmarksI think you get it

Just keep clicking next for proof the Core 2 Duo is the best CPU out there. Also the most expensive!