Let's explain this correctly and clearly:
6GHz dual-core does not equal 3Ghz quad-core at multi-tasking.
Today's computers applications vary greatly in how many cores they take advantage of. General websurfing, e-mail, videos, music, etc., essentially average users' applications, require a low clocked (the clock is the "speed" of a CPU) dual-core. Gaming is a more intense dual-core application, however, it has not been programmed to use four cores yet. Since it does require a lot of "speed" from the CPU, a gamer would want to buy a high clocked dual-core. Lastly, we have the quad-core processors. Among the applications designed for them are video and sound editing and encoding. These have been programmed to take advantage of multiple cores, so they would perform better in more cores rather than in more clock.
For example:
Computer A - 3.4GHz dual-core
Computer B - 2.6GHz quad-core
Computer A would perform better at gaming, while computer B would perform better at video encoding and editing. Both of these would perform extremely well in your every day applications because they both exceed the requirements for them.
There are also differences between each quad-core available and each dual-core available on the market, but this would take you into their architecture and specifications; something that is deeper than what you asked for, but still important to be aware of.