crazy: actually, Intel's does more work per clock cycle, so despite the lower clock speed, the two are roughly equivalent in single threaded tasks. The i7 is a better choice, even if you don't overclock. If GHz was all that mattered, the Pentium 4, with stock clock speeds as high as 3.8GHz, would still be king for single threaded, but an E8400 will utterly flatten it in every way because of the more efficient architecture.
(Oh, and you can think of the i7 920 as a 2.8GHz CPU, since that's the speed it'll run most of the time anyway with turbo mode).