Will an entrance level Intel Core I7 920 be faster than a Core 2 Quad Q9400?
Still they have quite similar main specs the processors are quite different to compare.
At least you will run the processors on different platforms, say, X58 and X48 respectively.
On one hand for general applications you will have almost the same performance. I think the difference will be within some unnoticeable 10%.
On the other hand due to the improvements to the Core I7 arch (namely, Turbo Boost, Hyper-Threading, QPI, L3 cache and overall memory arch) for some memory intensive applications you may hope to see another 20%.
Are you building?
The Core i7 is based on a very new microarchitecture, named 'Nehalem'. Core 2 is based on the previous generation, the Core microarchitecture. From general principle, I'd go with the newer (i7) - you would expect moderate improvements.
Not that CPU performance should matter much, it's not a bottleneck in most applications.
I'd wait a bit until the Core i7 will mature, say a year.
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