Saying that the i5 or i7 is faster than a dual core is a very subjective claim.
YES - the i5 and i7 can perform more calculations per clock cycle, and as a result of this are CAPABLE of running programs quicker than a C2D processor of comparable speed. But lets just say you're in the following situation....
You have 2 screens set up, with EyeTV tv program running on one of them, it has 2Gb of RAM in use for the live TV buffer. You have Photoshop running too which is using over 1.5Gb in a large work flow.... So all in all about 4Gb RAM in use, when you consider the RAM for OSX and other background services...
A C2D mac with 8Gb RAM would cope fine with this, whereas an i7 with 4Gb RAM would probably come to a stand still. Not because of the processor though, the i7 would be sitting around wasting clock cycles waiting for VRAM from the hard drive to feed it at horridly slow speeds - because RAM is all used up. Whereas the C2D with plenty of RAM is purring along nicely, as it's being fed straight from the RAM, which is only half full....
For most people, a C2D may be enough power. The whole system setup affects speed. A fast processor is useless with hardly any RAM for what you want to do, similarly a SSD would give you quicker start up times and vastly increase general usage - without having to change your processor.
This is only an example to prove a point that having the fastest processor is not always the best way to a fast computer - it depends on your needs.