The i7 quad cores(8 threads) may times do out perform the sexacore AMD chips. I think what the issue may be here though, is each i7 core only has one 128bit SSE unit which is shared between both kernel threads.

You should try running an SSE miner on 4 threads and another 4 non-SSE threads. The i7 should be able to handle 4 sse and 4 fpu threads without contention.

The new Sandy Bridge Intel CPUs should not have this limitation as they have two 128bit units. But SB chips do support AVX which is even better than SSE. Once miners start to support AVX, then you will have the same issue with a 4 thread limit on a quad core because AVX is 256bit and it will require taking both of those 128bit units and using both at the same time. So again, only on thread per core may use both units at once. But until AVX, I would think 2 SSE threads per core on SB chips should work.