Nehalem is going to shine for anything multithreaded (especially past the 2 threads point) when compared to Core 2 Duos/Quads. I think single/dual threaded performance is going to be pretty close.
Why? Well, Nehalem gives up L2 cache in exchange for that IMC, and I bet that L2 cache is still faster than system memory with the IMC. Since the core 2s have more L2 cache and share it among two cores, then for single and dual threaded benchmarks, I think they'll compare very well. Past that, they'll be hitting system memory more and more (not to mention the advantage of hyperthreading in nehalem) and lose way more performance.