On single-tasked programmes, all Cores have the same speed per MHz, as you can check on forums for SuperPi benchmarks.
More precisely, Cpus with a direct connection to Ram have a 5% advantage.
So unless you have multitasked games or video encoding, take a dual-core with a high frequency. The C2D's Lga775 limits upgrade possibilities, this is its only drawback.
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Hyperthreading is better than described above. The Cpu has two sets or registers for data, state and so on, and feeds computation units (=Sse) with requests from both threads, tick for tick. This is how HT achieves up to 30% improvement on multitasked software. Far better than just improved context switching. But not equivalent to one core more, and certainly useless for single-tasked software.