AMD platforms are cheaper for what they offer but the I7 is currently the fastest processor if that's what you're after, if you're willing to pay shitloads of money for it. In my opinion it's absolutely not worth it unless you REALLY need totally the fastest thing there is, like if you're willing to pay multiple times more money for a processor that will unrar a 1gb file 0,3 sec faster.
The cheapest I7 costs twice as much as the fastest Phenom while the performance difference between them is minimal. Also AMD platforms are cheaper and more future-proof.
I'd get a Phenom.
I5 is more reasonable than i7 but also much slower than it and Phenom 2.
However all of these are so good, that they're an overkill for 99% of tasks. Even most modern games would struggle to use more than 25-40% of their power. Actually the most demanding titles can barely make full use of dual-cores from 2-3 years ago. Damn, some MODERN games don't even use 2 cores! There's a different story with GPUs though. You need a good one.
Imo the I7 is the response to those "I want the newest and the fastest CPU with no real reason, just because I want it to be better than everything else and I have my parents' money so I don't care for the price" kids. Same story as with dual-GPU configs. Probably less than 0,001% would really need them considering that modern higher-mid end hardware is still an overkill at the moment and will be for the next few years.