http://www.anandtech.com/bench/Product/88?vs=109
Performance is pretty close.
Price? $130 USD v $200 USD. Not only that, but the nicer intel motherboards tend to ask bit for USB 3.0 and Sata 6gb/s; though the H55's and H57's are very budget friendly.
The cheapest X58 is about $170. and the cheapest i7 is about $280. Let's look at some more benches.
http://www.anandtech.com/bench/Product/147?vs=107
Again, pretty even, the i7 870 has an obvious edge in gaming, but the X6 stays close in raw processing benches.
Prices you say? $180 and $280!
On top of that, intel is expecting you to get a new motherboard to use the latest and greatest(?) CPU's.
Naw, AMD's got better price/performance even with quads and beyond; the fact is that none of their quads coast as much as the cheapest intel quadcores.