More cores = more complexity = (generally) less stability overclocking, but there are chips out there now from both lots that do 4GHz easily. If all you care about is performance, the i5's and i7's are the ones to have. If you routinely use heavily core-dependent tasks, the AMD Phenom II X6 is a good chip. It's just not as fast as the i5's or i7's in lightly or medium threaded workloads. The right program can make an i7 scream in terror while the Phenom II X6 will be fine, though those apps are rare especially considering Intel made HyperThreading on their i7's work very well to their advantage.
The sweet spot right now is an i5 and overclock it. If you don't overclock, the i7 is great if you can afford it, especially the 2600 / 2600K