It depends on what the PC is being used for.
If gaming the higher clock speeds have a slight advantage.
For anything else the Quad is superior eg decoding, photoshopping, running lots of apps at once. The Quad will also offer more in the way of future proofing as new apps utilise more cores. In order to fully realise the power however you would need Vista 64 as it supports multicores much better.
If you intend to overclock, the Q6600 will reach 3.0GHz with ease, and many here reach 3.3-3.6GHz.
Its a good hobby to take up btw